Imagine that you need a medical for your work. You go to your doctor and say "All I need is for you to authorise these tests for me to give to my employer". The doctor has to authorise the hospital to take the tests which you cannot do.
Does this help?
yeah I think the second one is more common actually becasue if we say "need for you" that sounds like "we need something for someone".It's like saying "I need a medecine for my friend".It doesn't mean that we need something to be done by someone"
That's what I meant from all the beginning.