Messi doesn't need Xavi and Iniesta to give him assists. He DOES benefit from them though.
Like any forward, Messi needs to get the ball in dangerous positions in order to be most effective. Xavi and Iniesta make sure that Barcelona keeps possession until someone can get it to Messi in a dangerous position. If Barca had a much lesser midfield, they would undoubtedly struggle to consistently work the ball into the final third. They would lose it in the middle of the park a lot of the time, and Messi would therefore end up with much fewer opportunities to work from where he is most effective.
We saw examples of this with Argentina the last few years. The issue with his play for them was not that he was actually playing badly. It was that they did not have a midfield that could keep possession in the middle of the park and work the ball into the final third. As a result, they couldn't get the ball to Messi enough. In response to that, Argentina decided to simply play Messi in a deeper role. That way, the midfield could actually work the ball far enough up to consistently get Messi the ball. However, the problem with that was that he was no longer getting the ball in particularly dangerous positions when he was playing deeper. This made it really difficult for him to score goals, and while he played as an excellent playmaker, people expected goals.
I think this all goes without saying though. We all know that a striker is more effective if he has a more consistent midfield that can get him the ball where he wants to get the ball. It is 100% impossible to score 73 goals in a season without that; there just would never be enough chances. Messi has a consistent midfield, but so does Ronaldo, and so did virtually every player who scored anywhere near the amount of goals Messi is scoring. Every top goalscorer played on a top team.
Is Messi's midfield maybe even a bit better than other's were? Yeah, but in my opinion they are better not because they create more chances than other midfields like Real Madrid's. I don't really think they do. They are better because they keep possession SO much that it is its own defense too. That is, other midfields will keep possession long enough to work the ball into the final third, at which point they will get it to a forward in a dangerous position and let him create. Barcelona will keep possession even longer than that, stopping the other team from having a chance to score for a while, and THEN getting the ball to a forward in a dangerous position. The difference is not that those other teams create fewer chances; it is that those other teams play at a higher tempo that allows the other team to get the ball more and have more attempts to score. But that added defensive value doesn't add any effectiveness to Messi.
