Heavy Flanking is an optional rule (DMG 251) that allows allies work together to defeat an enemy. As long as two allies are on opposite sides of an opponent, both get advantage on their attack rolls.
That is huge!
This is why coordination and battle tactics make a difference.
Pro-tip: any sort of familiar, companion, or allied summoned creature counts for flanking!
Pro Pro Tip: Allies can keep circling around an enemy, and as long as they don’t get more than five feet away, they won’t incur an opportunity attack.