A project can use marketing. Getting the project known out there. Get the message out that this project exists. One way to do that is to produce and get stickers out to people. Stickers are surprisingly popular among certain crowds and sometimes it seems that the hunger and lust for small logos with glue on them cannot be satisfied.
As a maintainer of a project you will at times be looked upon or asked to provide stickers, t-shirts, hats or other merchandise with your team logo on them. It is hard area to actually make money from, but they are products that can help create a bond and team spirit. It is fun and encouraging that there are a bunch of people out there who want nothing more than to put the symbol of your project onto the cover of their laptops.
One hard to circumvent problem with merchandise is that they are physical goods so it immediately gets expensive to send out to people all over the world. That is of course also why it is so popular to hand out stickers on conferences and meetups.