What is the purpose of launch images in an iOS application?

Question

How important is it to add Launch Images to an iOS application? I assume that these are like a cached file to show on the screen while the application is getting setup, when there might otherwise be nothing to show until it is fully running. Is this right?

If so, what should a launch image be if your app might start in different states, or does this not effect return to foreground when multitasking? If it is only for the very first initial launch of the app, then the launch image should just be the same as whatever graphic normally appears when you first start the app, right?

Apple must not care too much about whether your app actually includes these, since mine never have.

Answer 1

Note: as Zev mentions below, the Apple HIG recommendation has changed since I originally answered this question. Apple now recommends that you take all of the static elements of your first screen and make that into your default or launch page. All dynamic elements are to be removed, to give the impression that your app is going through an initialization process -- which in some cases may mean that your default screen is simply a background from your app. Thanks to Zev Eisenberg for pointing that out, "splash" pages are not recommended.

The launch image will show while your app is initializing, it's a good idea to have one. If you don't have one the user is likely to stare at a blank screen while waiting for your app to load. I believe the recommendation is to use an image that looks like the first page of your app. Here is the reference:

http://developer.apple.com/library/ios/#documentation/UserExperience/Conceptual/MobileHIG/IconsImages/IconsImages.html#//apple_ref/doc/uid/TP40006556-CH14-SW5