If you are lower even if you win all placements, whats the point?

to reduce how low you go?

Winning increases your mmr, which means you get more LP from wins. Making you climb faster. But you still need to climb, even if its just a division. You can't just coast by. Riot wants an active ranked scene, and they will enforce it.

Some people start higher.

To clear out the cesspools that form at every V division

It's way to artificially increase the sample size the matchmaker has to work with before it assigns you a division. Otherwise your rank and LP gains/losses would swing wildly for your first few dozen games.

I only won 3 of my placements but it put me in gold 3 anyway lol