As I said before, milling is done for two main reasons: you need to reclaim a very bad surface (patched, crack sealed, change of grade etc), or reduce thickness of existing pavement.
For example, you have an old road which was paved over several times without change of curb. The curb became...
Oh sure, milling is mostly done for reclamation or when you have too thick of a pavement. Often (depending on a condition of existing pavement) you can clean it up, put a layer of tack coat (asphalt emulsion), and pave right then.
