Question

beside heavy processing, should multithreading mainly be used when you have a not quite responsive UI? or does it have other considerations?
How can I know if my application should have multithreading or not?

Answer 1

The use of threading in ios is to ensure hussle-free and seamless experience by the end-users.
You can implement thread whenever you want to extract some resource over the network such as parsing or data retrieval and you don’t want the ui to be affected as application would run on main thread and the web-operation on your custom thread.

You may want to use the thread when you need to have concurrent operations or simultaneous such as in game when you hae to have multiple animations on same object at same time.There can be quite a large number of scenarios which may need threading.

You may read Concurrency Programming Guide By Apple

and Thread Management

but threads may be an overhead in the application as it needs memory allocation and large operations on thread may affect the performance so use it when it can’t be avoided.

You can use NSThread,NSOperations to create threads .GCD is deprecated now.