Open source always free.
Why would someone prefer Lichess over Chess.com?

1. Lichess has free puzzles
2. Lichess puzzles come from actual games and are much more difficult & your puzzles rating there is closer to your actual skill (They use a different rating system, so you'd have to guesstimate it to elo, it's not that difficult though)
3. Lichess has free public studies that have a lot of information
4. Lichess is better for saving and analyzing games for later so you can learn from your mistakes
5. If you're reading a chess book you can make your own study and document what the book says

I agree with NyanUnderHel

1. Lichess has free puzzles
2. Lichess puzzles come from actual games and are much more difficult & your puzzles rating there is closer to your actual skill (They use a different rating system, so you'd have to guesstimate it to elo, it's not that difficult though)
3. Lichess has free public studies that have a lot of information
4. Lichess is better for saving and analyzing games for later so you can learn from your mistakes
5. If you're reading a chess book you can make your own study and document what the book says
Agree with you.
Currently I use lichess studies for my chess training and I am very satisfied with it.