MMR how it actually works.

I still dont get how the MMR works. I mean there are games that I am doing well in my lane, but apparently other lanes are doing terrible. Which concludes in me losing the game. Now in some games I might be for example 0/5 at start of the game due to camping but in the end I will win the game cause rest of the lanes do their job. Now in the majority of the games I get a lot of people trolling, inting, not even farming at all or AFK. But I notice that in those games the LP that I lose starts going higher and the LP i win after 3-4 games is lower. So to sum it up, the MMR is based only on win and loses? I mean why try to be good in games that for example I have 0/5 mid, 0/7 top and 0/8 jungler? Is there a point since all concludes to win or defeat? I would appreciate some information on it.
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