The advantage would be to bear down on the few poor souls standing there to hopefully push through the enemy line with irresistible mass. Pushing through a forest of pikes or spears doesn't look like a very lucrative prospect for a horse though, I don't know how they'd train them to ignore that.

Getting behind the line would seed confusion and panic, as it divides the enemy (and everyone hates to have enemies behind him). I assume infantry would rush to fill in the open space, completing the division.

Anyway, mass bonus probably won't work because in RTW the cavalry usually stops at first contact with the enemy, then saunters forward to fight. You're better off just going around since the AI doesn't seem to care much about keeping some troops in reserve to ward them off.