Another thing it does is helps people realize what power they have, even if one day of boycotting has zero impact on the economy or businesses. It gets those people who are participating started taking action, and thinking about their actions in the context of politics.
It’s a very easy first step, and if people find that they can do a day, maybe they’ll be okay with trying a week next time, or maybe showing up at a town hall seems easier. This is arguably more about getting people involved in the movement than actually sticking it to the corporations/oligarchy. That will come. But asking people who live paycheck to paycheck to boycott corporations for more than 2 weeks would be a huge ask without building up to it first.
Gotta start somewhere with people. The point is that anyone can do this, and it’s easy to do, but it isn’t really any more difficult to show up to a town hall. And while yes, you and I can (and probably do) take larger, more effective steps, longer boycotts, etc. We need numbers, and that, I think, is the real value of this.