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 dunno. Lemme ask the homeless guy with a bag of McDonald's.
People also volunteer to feed homeless people, so I don't see what point you think you're making.
Mutual Aid is good. Supporting the charity industry just lines the pockets of capitalist wives and failsons.
Great, let me know when mutual aid blood donation exists.
Seriously. Whether or not it counts as "mutual aid" and whether or not you are compensated (which, yes, you should be), if you can and are allowed to donate blood, it's helpful and does one no harm.