What you said and what I said are both true.

N95 masks will protect you, but the reason good and well-adjusted people want others to also wear them is to protect everyone else, especially immune-compromised people for whom a COVID infection could well be life-threatening.

And of course it's also a numbers game. If even 10% fewer people get infected, across a large population that's a quite significant number of people who aren't clogging up hospitals and calling out sick from the work force.