When should one copy the proof of known results in a mathematical PhD thesis?

When writing a PhD thesis on mathematics one needs to quote many results by others, such as

The following theorem was proved by ABC in [1].

Theorem. Bla, bla, bla...

My question is when should one include a proof of the result in his/her PhD thesis, if it is basically the same as in the reference? I do not quite see the point of copy-pasting the proof by others. Of course if one has a completely different proof of the same result, it is probably suitable to include it.