In-combat healing can also be useful when the enemy is coming in "waves". For example, you just beat one group of foes, but there's another group headed your way that will be on you in a couple rounds, or a minute, or whatever. You can take that short breather to top up the most injured PCs and prepare for the next fight, but won't have time for one of the lengthier out of combat healing options.
(In many situations, especially in buildings and dungeons, combat could/should attract more combat. It seems like a lot of combats are run as though they're in a vacuum. You enter one room in a dungeon, fight whoever's inside, clanging around with weapons and armor and blasting spells, then once that's done, you move to the next room over to fight whoever's inside and who apparently hadn't noticed the fighting and explosions from the previous room.)
In-combat healing is also useful in situations where there's a time crunch. For example, the bad guy sends his henchmen to fight you while he is trying to get away with a captive, so you don't have the opportunity to say "Time out, Mr Big Bad... We've defeated your minions, but we're hurt, so now we're going to take 10 minutes/1 hour to heal up with Prayer of Healing/a Short Rest before continuing our pursuit!" Or maybe you're trying to barrel through a stronghold to stop a ritual that's in progress and that will be completed in 10 minutes.