iPhone apps start "instantly" because the os zooms a pre-drawn picture of the application onto the screen. Often it looks like the frame of the app but without the content.
If you have a dynamic picture you could save a snapshot of what's on the screen when the app was closing, save it, and use that as your default image next time it starts. i.e. the last email you were looking at. It makes the app feel even more responsive when starting up because straight away you see what you saw before you closed the app.
There may be other uses but that's the one that jumps immediately to mind for me.
