TL;DR — the practice saves lives, even if not yours.
A risk that’s trivial for one person is a real concern when it’s suffered by 300,000,000 Americans.
How much modest personal discomfort and inconvenience is it worth, to save devastation somewhere else? Especially when you don’t know the people, don’t know where, and don’t even know for sure it’ll happen? People donate dollars and volunteer time when earthquakes and hurricanes strike. Standing in cold flood muck all night stacking sandbags by the river is pretty uncomfortable and inconvenient. Masks are a lot easier.
I’m not a public health expert, but I can do simple arithmetic. I’m going to make up some numbers here. If you have numbers you like better, use them. It will only strengthen the logic. If you are numbed by numbers, just go back and reread the TL;DR.
Suppose in a given encounter you have only a two-in-ten-thousand chance of getting infected. If you wear a mask that’s, say, 50% effective, it cuts your risk in half. Masked, you have only a one-in-ten-thousand chance. Big whoop, from pretty negligible to even more negligible. Why bother? There’s practically no chance it will help you.
Yet, imagine 300 million maskless Americans, each with your 2/10,000 chances. That’s 60,000 new infections right there. (Confession: I checked the simple arithmetic with a calculator.)
But if all 300 million wore a mask that halved their already small risk, that would halve the number of cases nationwide. That would be 30,000 fewer sick people spreading around COVID and concern.
“But,” someone might respond, “I live in a small town, not in a big den of contagion like NYC, so we don’t need masks here.” However, the same general logic applies, with large numbers times small ones. There are more than 16,000 small communities in the US (with population less than 10,000 — I looked that up instead of making it up). Even if they each have a 99.9% chance of making it through unscathed, that’s still 16 towns that would be hit. What if we could spare eight of them?
So please, wear your mask. Public health is practically a civic duty.