Playing against yourself

What can you learn by playing against yourself?

Do you learn more or less than by playing against others?

You mean playing best moves on both sides in a solo game? I think one good thing it helps with is your appreciation that your opponent has a plan as well and in a real game, you'll probably not play with blinders on and ignore what your opponent is doing.

Though your second question is easier => Of course playing against other human beings is better. I'd follow the progression from worst to best in terms of value to your chess playing.

1. Playing against a weaker player. 
2. Playing against an approximately equal-strength player.
3. Going over the game post-mortem with equal-strength player.
4. Playing against a stronger player.
5. Going over the game post-mortem with the stronger player.

and best of all => 6.Having a good coach go over any game you play and identify mistakes and remedies for you. 

Of course, playing weaker players is good in one sense => because you learn to practice winning won games, work on technique and ALSO practice NEVER underestimating them. Though you can't really make gains if all you ever do is beat up on people weaker than you.

When you play against others, they'll point out things in the position that you don't necessarily notice on your own. They may play a move you don't notice. This trains you to try and notice everything better than play against yourself.

if you play chess againest yourself you learn how you would react to the moves that you would play and then you could change them

You should be doing that in your head before playing the move, even when playing an opponent. That's the essence of chess - thinking of your opponent's possible responses before you move.