I think the last thing you described is the intended purpose. Imagine the following scenario: Knights in wedge formation smash through the enemy's lines and pull out beyond. Your infantry engages the enemy from the front while the knights charge in from the rear, this time in line formation
= victory.
The "smash-through wedge" seems to be the direct way for heavy cav to get in the enemy's rear. Horse archers and light cav are good flankers, but knights are simply too slow for that.