>>528016446
usually it boils down to weak early game champions that have good wave clear and thus consider it being safer to kill the wave behind the enemy tower, away from the threat of being harassed in the wave by the enemy. Since your enemy needs to sit under their own tower to last hit, they have to choose between harassing you (and losing gold/exp) or letting you scale out of your weak early lane phase for free (and eventually being outscaled, but not falling behind themselves.)
Champions like Sion can also abuse the fact that his wave will always be under the enemy tower by walking up, proccing Demolish and slowly taking all the plates without ever having his own tower be threatened