When is it optimal to use a standard Action to knock an enemy prone?

I'm imagining a situation where a monster is already standing in melee range of the party's tank, and the tank and the monster are standing in place and duking it out for multiple rounds. The monster has no particular use for its Move, so the monster can afford to use its Move to stand back up each round before attacking. That means the only mechanical benefit (that I know of) from knocking the monster prone is that your party will gain advantage on attack rolls against the monster for the rest of the round.

One of your party members probably wants to cast a spell that doesn't rely on attack rolls. One of your party members probably already went earlier in the initiative, i.e., before the monster's turn. So maybe you have three party members who can team up on the monster. PC #1 trips the monster, knocking it prone. PC #2 attacks the monster with advantage, and PC #3 attacks the monster with advantage. Is that really better than just having PC #1, PC #2 and PC #3 all make regular attacks against the monster? Even if PC #1 has a 100% chance of knocking the monster prone, it seems like you'd need some pretty specific to-hit chances for the math to work out in favor of tripping an opponent. Of course, in a real scenario, usually the monster will have at least some chance of winning an Athletics check or something like that. So why (or when) would you ever trip or grapple a monster in combat?