The quick answer is: it makes it possible to work on project full-time. Only 1 of 2000 makes a donation https://darkreader.org/blog/500k-users/
Apple is the only place I have to pay to develop. Apple is a luxury product. Apple users can pay luxury prices for my time.
At the same time Chrome Web Store is not suitable for paid apps, today it looks like a big dump: it is filled with outdated and poor made extensions. Raking system makes good apps hardly discoverable. Also paid Chrome extensions work only in 36 countries. Dark Reader owes its popularity to Hacker Vision extension that became paid some day. Maybe that's the reason why some developers prefer monetizing their extensions by selling users browsing history.