No he didn't have to, but he did. Once he's played advantage, deliberately or not, it would be against the rules to later give a free kick and remove a legitimate goal.
I have a vague, probably imaginary memory of it happening in the early days, but certainly not for a long time. Which then begs the question as to the point of them going to the monitor at all. If VAR want the ref to check, then by definition it's not clear and obvious. If it is clear and...
It really depends what clear and obvious means. Does it mean that
a) the foul by Hughes is easy to see?
b) once you take the time to look closely, it's undeniable that it is a foul by Hughes?
It is a fact that it was a foul. It was an (understandable) error by the ref. Those are facts, so it's...