Buying time.
Time was according to Napoleon I the one thing you could never get back if lost.
example. loosely inspired by 1st Minnesota
You need 10 minutes to get a brigade of 2000men to a critical position that is good for defense from both sides.
The enemy is attacking with 3000 men, and will get to the position in 7 minutes.
If you do nothing, the enemy will take the position first and your outnumbered brigade will be stopped with heavy looses.
So you find 200 men, they attack the enemy and he "loose" 5 minutes to destroy the 200men.
You now get your 2000men into the good defensive position, and they stop the enemy and inflict heavy casualties on him.
So even with the sacrifice of the 200men, you held your position, and inflicted more casualties on the enemy than you lost.