What is the Purpose of Hiding Our MMR and How the System Works?

^ serious question, I am sincerely asking why it isn't public knowledge.

I was thinking about how MMR would work once the Role Ranks are introduced and how other websites such as OP.GG will be able to calculate our estimated MMR. Then I was wondering why it is kept hidden from us in the first place, there must be a good reason otherwise it would be visible.

Its not like its actual useful information, but it would be nice to just check it. Might help with understanding why you are being matched with certain ranks though.

EDIT: Bit of a misunderstanding here, I am not asking why we have our current Tier/Division system over an ELO system. I understand that, I know it used to be an ELO system and I understand their reasons for switching.

I am asking why we can't ALSO see our MMR and why we don't how the system actually works. We go to other sites not associated with Riot for approximations of our MMR, most of us do it even though its not useful information simply because it is nice to know. So I am wondering why Riot just doesn't give the exact number so we don't have to guess.

EDIT 2: A lot of people saying that if it were public knowledge it would be abused, but I am fairly certain that MMR is only affected by winning or losing, not personal performance. You go 15-2 and win you get the same raise in MMR as if you went 2-15. The only thing I would like clarified is how they decide to match up MMR's and how it calculates how much you gain/lose.