More for helping me to remember...
One good reason, at least during development, for putting your web application/service in its own app pool in IIS? Recycle...
There are times when you'll want to recycle, normally done by restarting IIS, to make sure it updates and recompiles against your current dlls.
But in a shared environment it's not very polite to restart IIS and mix up your fellow devs.
So throw the app in its own pool, problem solved, as it were. You can restart just that pool and not have to restart the whole IIS service.
Again pretty minor, but useful to remember.