Dll Injection - What is possible with it?

Question

I was browsing the internet lately, when I stumbled upon Dll Injection.

I think its an interesting subject but, I have no clue what the purpose of it is?

I have

Answer 1

There was a very good project done in Microsoft Research about this very subject, focussing on intercepting calls to WinAPI functions by using dll injection.

This is the link, it should keep you busy for a while: Detours