Easy: So you can see what effects the changes have right then and there. Having to reboot between kernel tweaks is the winblows way...

Another way to say this is that Swappolube's default settings are not necessarily the correct settings for how you run your particular N900, so you may want to tweak further yourself. Seeing the effects immediately upon committing them to the kernel helps tremendously and is the whole point of being able to change kernel parameters on the fly, whether on a semi-embedded device like the N900 or a full-blown PC (where all of these settings also exist, but I personally have yet to play with them).

That brings up a good question, actually: how come there isn't a Swappolube for all the other Linux distros out there?