Design patterns make learning a new API much simpler. Once you've learned a particular pattern, you can understand all of the classes that use it a lot more easily. If you understand the target-action, delegation, and model-controller-view patterns, then you will find Cocoa very easy to learn because these are used everywhere.
A few others, such as the designated initializer and singleton patterns, are also used a lot in Cocoa but are less important.
If you design new Objective-C frameworks, then make sure that you use these patterns when appropriate (and don't use them at other times), and other people will find your code very easy to use.