Yeah sorry I forgot
It's just a donation
Well, it is a donation. Donation, is when you need to donate, and of'course in this case you need money.
And Lichess.org has made it for people who are donating and for people who wants to donate, and if you want to donate as I said of'course you need money,... After all, it is your choice: Donate or not?..., and if you want so, it is of'course worth to donate money.
Lets not be detailed, Lichess.org is free, and donation everywhere worths money.