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

Answer 1

how about if app is starting immediately and no time passed ? what if you see (or cannot see) for a 0.1 sec launch image ? would it be still useful to have two images in main bundle that will never be seen or used ?

Answer 2

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