I can understand why one would want to donate if you find the product useful, but donating because it’s free doesn’t make any sense to me. Could you elaborate?
Lichess is not for profit, so donating to Lichess means that my dollar goes "farther" for infrastructure & helps subsidize the website (which has many features) for other people who may not be able to pay. The main developer only pays themselves $56k a year, when they could easily be making $300k+ in the valley.
Chess.com is for profit, so they have to maintain some profit margin and lock features behind paywalls to incentivize people to pay. The free experience is worse than Lichess.