Well, I started to reply but I'm a bit long-winded, so here's a crash course in imaging for a broader range.

In pure theory, a light meter has no bearing on DSLR, because they include self-metering and auto-exposure. In practice, it can.

SLRs, however, don't have that (typically). Meaning you need to set aperture and shutter speed depending on film speed and ambient light.

In a camera, it starts with ambient light, we'll call that 0.

Then you have ISO, Aperture and Shutter.

Aperture.
For an aperture equal to 1, a quantity of light hits the sensor (when the size of the hole is the same as distance to the sensor, that's 1). As the radius of the aperture (iris) gets smaller, less light comes in, but because surface area dictates light not radius, the light that comes in varies with pi multiplied by squared radius (formula for surface of a circle).

As a result, you get those eff numbers, called "stops". Each "stop" means that light coming in is halved. But it is measured in radius, not area, so the next stop is not 0.5, but f/1.4. I'll skip the math. Then, the next stop is f/2, then 2.8, then 4. Each stop, light is halved. At f/1.4, 50% light reaches the sensor. At f/2, 25%. At f/2.8, 12.5 %. At f/4, 6.25%. Et caetera.

ISO
The film has sensitivity. The more sensitive the film is, the less light it needs for the same image. And, you guessed it, it works in multiples of 2. ISO 100 is going to be my standard. There's ISO 200, 400, 800, etc. Values over 3200 are rare. Bigger numbers mean faster. Remember, the faster the film is, the more sensitive. The more sensitive, the more likely it is to pick up gunk. "faster" film also means more noise, less quality.

So, at light zero and F/1, ISO 100 gives you perfect image. Also good values are: F/1.4 (half light), and ISO 200 (more sensitive). F/2 and ISO 400, and so on. Film also goes the other way, ISO 50, for example. Not much, though.

The lower the ISO, the higher the light requirement. Also, it takes more to make an impression, so it tends to give you the best quality (noise to ratio).

Shutter Speed
The third component is exposure. Exposure is the time you expose the film (or sensor) to the light. For example, 1 second. It is also known as shutter speed, or, simply shutter (in context).

So at light 0, ISO 100, f/1, you need 1 second to get the pic.

If you have ISO 100 film but it's too dark out, the lens is F/1 big, so you need 2 seconds of exposure.

Unsurprisingly, shutter speed also has every-factor-of-2 settings, but since it's the most controllable feature, it usually has finer control. It's easier to fiddle with time rather than aperture (for most cameras, but not all).

So, you have this combo: Ambient light is fixed. in a SLR, so is film. You are left with aperture, which is limited by the lens (especially in the "open" corner) and as a result, set shutter speed to compensate.

Problem is, while aperture, ISO and shutter all have numbers, ambient light is natural. So you need to find out how much light there is.

Remember that 0 back there? That's daylight (ish). It goes +1, +2, +3 as it goes brighter, and -1, -2, etc as it goes darker. Unsurprisingly, it's called EV, for Exposure Value

Every time it goes down, you need to ante up exposure. 1 second at 0 EV, 2 seconds at -1, 4 seconds at -2. And the other way around (1/2 for +1). IIRC.

Back in the day, people had to resort to good old eyesight to get these settings, and things like sunny 16 rules applied "On a sunny day set aperture to f/16 and shutter speed to the [reciprocal of the] ISO film speed." (wikipedia)

A light meter is a gizmo you gold in your hand, point at your subject and you read "+1.3". There's your picture right there if you have a table or you learned it.

Additionally, people walked for a day to get to a picture, clicked the button, the walked a day back, then either manually developer the pictures or sent them to be developed, both at cost. Light sensors made sense.

DSLRs have a sensor. It runs when you half-press the shutter and, depending on setting, adjust one of more settings to get the best picture because it knows how much light there is.

There are exceptions, however. For example, you could shoot in harsh conditions, like trying to see something in the shadows in bright sun. Camera guesses a setting based on sun, so you switch to manual and make it capture more light, "burning" the sunny side, but capturing the dark.

Also, each setting has its side-effects. Aperture affects depth of field, for example, and can take items out of focus. ISO speed affects quality. Shutter speed affects motion artifacts, so it's not like all settings are equivalent. For example, shooting a car on a track with high-aperture high speed makes it stop in its tracks, while a low aperture, low speed makes it leave a trail, but also brings the people on the other side of the stadium into focus.

Some people like to shoot manually, for some reason. The rest of us (and the rest of the time) use priority modes. I won't go into that, as DSLRs aren't exactly the focus here. Heh, focus.

However, there are points where manual shooting makes sense:

* High dynamic range and amateur-class camera
* High dynamic range and no PC in between for HDR
* Manually over/under exposing an image in harsh conditions above camera ability
* Art-like shots that require dramatic exposure (night shots in daylight)
* Paparazzi mode, where there isn't time for a DSLR to meter
* Camera errs. It's rare, but metering is usually done on spots/segments, rather than full multi-megapixel image, so it's possible for a bright spot to fool it. Usually, one does a meter-lock technique, but manual is better (if slower)

Also under camera-errors there's reflection. If you shoot shimmering materials, the camera might go haywire, so a light meter with a dome can be used, to measure light diffused, rather than spotted.

In large SLRs that have big film (plates), those are expensive plates. You need to make sure you get correct settings as you can't just go around burning plates and developing them just to find out you've missed. That's why light meters are widely used with large cameras in studios. You would think studios have controllable light, but light reflects depending on subject.

Though if you have time to walk over and use a meter it's safe to say you're not doing anything dynamic (at that time).

Because DSLRs have such good tables and sensors, it's usually easier to fiddle with exposure compensation. What this does is, once the camera settles on a setting, it shuts up and uses a setting 2 EV lower than what it guessed. Or higher, or what you told it to do.

There are tons of thing not covered here, like priority modes, Paparazzi mode, etc, but basically that's what a light meter does, and why. It allows you to gauge ambient light so you can set manual stuff on your camera.

I've bored you enough.