Why use LP at all?

Alright so the other day I submitted a ticket into riot support asking how exactly the amount of LP you gain/lose in matches is calculated and to my surprise I actually got a decently in-depth answer. To sum it up the system is designed so that each rank and division is designed to coincide with a certain MMR. Depending on where you are in relation to this MMR determines how much LP you gain. If you are above the mark you will get lots of LP for winning and lose only a small amount. This is because the system had deemed that you are better then the division your in and will force you to climb. On the other hand if you are below the set MMR you will win only small amounts of LP and lose much larger amounts of LP for a loss. This is because the system deems that your deserve to be lower then the division your in and will try and force you to fall.

With this all being said it raises the question of why we even have LP. Why not just get rid of it and have your MMR determine your rank plane and simple?

What are your thoughts?