If you use third party iOS apps, i'm quite sure you will have seen something like this dialog. It was probably pretty annoying.
It sensible to wonder why developers would put such a user hostile interruption into an app, could it be that important for them to have their ego stroked by people telling them how wonderful their design and programming is?
Lets look at some data, here is what happened to RSSRadio downloads after the 4.0.1 release and the app rating was reset to zero:
Downloads dropped dramatically.
Now thats not ideal from a revenue point of view, but its actually a lot worse if it continues for any length of time. The App Store® uses downloads and ratings not only to place apps in top lists but also to place apps in search results, a prolonged period of low downloads will cause the app to drop in search results which will cause downloads to drop further which will... well, you get the idea...
Not having that magic 5 ratings which get your stars to appear could easily turn a viable app into a not viable one. Not a great incentive for developers to release a new version is it?
So, when you rate or review an app, you doing so will help keep an app you use (and maybe rely on) developed and maintained on the App Store® and it only costs you 20 seconds of your day. Thats not such a bad deal.