Applications are sometimes "siloed", even in frameworks that support multitasking. That can spell disaster when multiple apps want to make use of the same resources. Even if they launch one at a time, that doesn't prevent conflicts if there isn't at least some low-level awareness of each other's existence. By that I mean something like (example only): "Oh, I'm not the only app on the device that can interact with media. Fine. I won't lock up any media resources or leave anything of myself behind when I close that could interfere with the operations of one or more of my siblings".

I'm oversimplifying, but only because we have a diverse audience here. Application awareness should be the default behavior in any environment these days, now that we're long past basic DOS.