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.
it's stigma from the aids crisis and in the past due to the stigma against being gay, gay men were forced to engage in risky sexual practices like sex with near strangers in order to have sex at all. That's a big part of why the gay community was hit worse by aids than the straight community