Like I'll grant that voting for the president probably doesn't really do a whole lot. (IDK. I am Canadian and we have a parliamentary system.)
But voting for the "downticket" or lower-tier offices and helping competent, principled people get their foot in the door has the potential to change things decades down the line. You vote to shape the establishment that activists 50 years from now will be dealing with.
I figure a lot of people just want to do something that changes things right away, for them personally. And in that case they may as well stay home. But if you give a shit about future generations and if you are okay with your actions not having a large or immediate payoff, voting and GOTV efforts are eminently worthwhile.