When and why to use performSelector - iOS

I have been trying to figure out why and how to use performSelector. I did come across https://developer.apple.com/library/mac/#documentation/Cocoa/Reference/Foundatio n/Protocols/NSObject_Protocol/Reference/NSObject.html
However, I am not able to fully understand it.
Would anyone be able to help me out in explaining this?

performSelector is useful for loosely coupled code. Useful also as callbacks from code that executes in background threads.

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