1. It is free!
2. The community is friendly! (almost)
3. There are no adds!
4. You are improving your memory!
5. YOU ARE IN TOUCH WHITH M A G N U S C A R L S E N !!!!!!!!!!!!
You don't get robbed by other chess sites
chess sites can't be great without horsey
There are no features hidden behind paywalls and stuff.
I don't even like chess but lichess is so great I keep hanging around.
On a side note this thread should probably be in the off-topic section (instead of all the crap that's normally there)