Different things work for different people. As others have stated, the closeness and bonding that comes with being good friends with your teammates can definitely help with keeping the vibes good, making sure everyone is comfortable in communication, and keeping the team together long term.
There are reasons why you may not want that though. Similar to how it’s generally seen as a risky idea to go into business with your best friend or become roommates with your best friend, becoming so intertwined can be a problem if just one aspect of the relationship isn’t working. For instance, maybe you want to play the game 7 days a week and grind all the way up to top level, but your friend isn’t that serious about it and only wants to practice once or twice a week. It’s not fair to have someone with different goals than you hold you back, but it’s also a shame if leaving that team also means leaving the friendship. Some players, especially ones who aren’t as mature, may have trouble differentiating you breaking up from the team and breaking up with them.
I find it’s not very hard to keep in touch with former teammates that I vibe with even if we’re not great matches as teammates anymore and when seeking a team, I’m just looking for a group with similar goals and views on the game who I can feel comfortable around, but I don’t necessarily need them to be a big part of my personal/social life. Kind of like coworkers in a way.