whats the point of pre season?

Riots way of seeing how bad they suck at balancing their own game before the fuck it up even further during regular season

Nothing much really. You could slightly increase your MMR but thats it. Point of preseason is to test and balance new changes because biggest changes come during preseason. It also allow players of highest ranks to take long break from ranked without fear of being demoted (challenger/master players). Also its usually most fun to play during preseason because there isnt much you get to lose so you can freely try new stuff in ranked.

wins and losses still affect your MMR

the point of preseason is to introduce big changes to the game while people aren't caring much about ranked so riot has time to refine balance a little before the ranked season starts for real.

There is no visible reward for playing ranked during pre-season. Your rank will get resetted at the start of season 8 and you will have to play 10 placement games again. The only thing that rankeds during pre-season do is changing your MMR.

It's basically a large scale PBE so Riot doesn't have to listen to anyone during actual testing