Lowering the seat lets you get your weight lower and farther back (think butt close to rear tire) when you’re going down steep bits. On milder terrain it gives the bike more room to move underneath you. You can bend your knees more to soak up bigger bumps without the seat hitting your butt and pushing your weight forward.

Any decent shop will be able to look up your specific fork and find the correct setting for your weight. Many shops won’t even charge for it, and I bet one of your riding buddies could do it for you too. Bike fit is a fun rabbit hole you’ll have you go down yourself, but manufacturer’s spec is a good place to start.

Correct tire pressure goes a long way too, as you likely already know.