I would estimate Barcelona as a whole is >20% better with Dani Alves on the pitch than with the current alternative. I say it is second to only Messi in terms of one player being able to make a difference.
https://www.youtube.com/watch?v=LKsBKQkGIDk
It is not mostly what he brings in terms of good crosses, 1v1 ability and so on, but it is about his calmness and trust. I see this when a former pro plays with amateurs, the time on the ball, the constant upwards head, the way of tempting pressure in order to play through it. Just check his first two actions in that clip - these things make your team more comfortable and the opponent much less comfortable, it influences the entire game and lets your team get less irritating pressuring from the opponent and more time on the ball, after the opponent finds out the pressuring isn't working.
I thought he was less good from the midfield, but in the clip he played very well with the ball at least. Puig found him nicely with horizontal passes from half space to half space twice there, it was a bit reminiscent of Xavi - Iniesta to be honest (I don't think Frenkie and Pedri for example found each other like that even twice in a game a lot of the time)
But I think he was out of position a lot, especially defensively. I want him as a RB, especially considering how the squad is balanced...