whats the point of towers?

If Nasus is three shotting towers you either let him free farm for most of the game or have been playing for a rather long match. He's a late game champion, if you play to his strengths when he's not on your team you deserve a loss.

the outer turrets are here to protect you in early game while the remining turrets act like figureheads the psychological effect it has in slowing down the enemy progress toward your nexus is pretty dam good

also they can kill small/medium sized minion waves