Project based learning

A teacher recently asked me if after all the projects I have students do, how I then prepare a test so I can grade them. 
I thought this was a bit strage: why do I need a test to prove they can do something if they've successfully proven they can do it?  For example, I have teachers write and teach lessons, I don't then give them a test on writing and teaching lessons.
Project-based learning is basically the idea of finding a real-world task, ideally one that combines several skills such as language and math, even more ideally one that will be shared with others, even more ideally one that will make a difference in the world.