I have seen many pro players dropping mules after and during battles. Sometimes they use scans instead.
This seems counter-intuitive, I mean they could get a ton of minerals for that.
Why do they do it? MarineKing does that very often.
There are three reasons that I’ve seen mules dropped away from the mineral line in competitive play:
By far the most common reason is that a Terran player has decided that they’ve won the game, that it’s obvious that they’ve won the game, and the other player should have realized they’ve lost and gg’d out already.
Throwing down mules away from your base in that situation says “I’ve won so big, look at me throwing away resources, because I don’t even need them to crush you completely.”
It’s considered impolite or (in the common SC2 phrase) “bad manner”, hence the name.
Forcing the enemy to shoot their own units
Facing off against another Terran, it’s possible to cause the enemy’s tanks to kill each other by landing a mule in the middle of a group of sieged tanks…the tanks target the mule, and kill each other with the splash damage.
Repairing mechanical units
When an important mechanical unit or group of mechanical units (most often a Thor, but sometimes a group of tanks) is severely damaged far from the players base, that player can repair it by sending down a mule, without having to wait for an SCV to cross the map, or retreat the unit(s) back to their home base.
If the unit(s) repaired are important because of their strategic position, repairing them can be more important than the loss of minerals due to dropping a mule away from the mineral line.