This of course only applies in areas where blood has not been commodified. But if you live in a place where the medical system hasn't been completely corrupted by capital, you should do it.
And yes do it for others, but do it for yourself too. It lets you, for half an hour every few months, exist in a totally profitless environment where members of the community voluntarily gather to give to other members of the community they will never meet. Do it to remind yourself how many people on your community who don't identify as leftists participate in something like this.
Giving blood is Kropotkin's coast guard ideal.
I'm not saying giving blood is praxis, or that it will bring about the revolution, but it's an opportunity to temporarily escape our neoliberal hellworld.
I like to refer to my donations in terms of how many milk jugs it would fill. I do this solely to gross out my girlfriend because I'm a massive child. Still I'm going to hit 8L (2gallons) next donation :arm-R: 🩸 that's a lot of breakfast cereal