The sooner this happens, the better for the player (if the ally is controlled by the AI, of course). Because then you can capture the former cities of the ally and use them for your own purposes. But you can't do this directly, according to the restrictions built into the rules. So, weakening the ally is in the player's interest if they are playing to win...

The benefit of having an ally is small - they reveal the map and visit obelisks, which, if I'm not mistaken, also become "visited" for you (but you can do this yourself if you recruit heroes in the ally's castles)... So, that's it... :)