Have you seen all the pads and equipment that Umpires wear ? I think they do that to steady themselves in their crouch. After all, most Umpires are a little older and not as fit as the players. I played Catcher for a few years and it's hard on the knees. They're just using the catcher for balance to lean in close and make the right calls.
I think it's so he can have a close up view of his line of sight. Umpire must make calls over the home plate, have a straight line of sight, and the catcher is in the way. He needs him to keep from falling over home plate.
a *wink wink* baseball bat fetish.
I cannot find a precise answer to this question.
From what I have looked at on the internet, it seems that catchers don't like umpires who do that, and that umpires seem to hold other umpires who do it in less that high regard.
If you want an overview of the issue, this is a good start:
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