Running the index costs bandwidth, storage and electricity; the client costs time. There's no subscription, no ads, and no plan to add either to any of our products. If you'd like to help cover the running cost, this is the only way we take money.
We accept Monero by preference, it doesn't tie a donation to an identity the way most other options do.
If Monero isn't practical for you, ask on Telegram and we'll figure out an alternative.
Donations are optional and change nothing about what data is kept. See the no-log policy for that.