It is, especially in combinations with grapples
Knocking somebody prone already makes it a lot less likely that they will be able to bother your squishy friends, as they'll need to use half their movement speed to stand back up, and if they don't stand back up they'll have disadvantage on their attacks, and allies in melee combat will have advantage.
That in itself is pretty darn good, but there are ways to shut down enemies even further. If you also grapple them while they are on the ground, their movement speed will become 0. This means they don't even get the -chance- to stand back up, they will need to either attack with disadvantage or try to break the grapple somehow, they don't even have the option anymore of using half their speed to get up.
How good this is depends a lot on what you're fighting. If you're fighting a group of 20 goblins, doing this to one of them is really not that big a deal, you'd be better off just hitting one and most likely instakilling it with your attacks. But if you're fighting a big boss monster, it suddenly becomes extremely debilitating for the monster.
I've personally done it with a Young Green Dragon, which is a lot less scary when it can't fly, can't reposition itself for its breath attack, and either has to use an action to escape that grapple, or resign itself to having disadvantage on all of its attacks.