When you have to play League of Legends without your friends it can be one of the worst experiences ever. So many times when a teammate does not get a role he or she does not want it will lead to a horrible game. This does a great job of making sure it that does not happen.
I main support so I rarely ever find a problem of getting my role as support, but too many times in solo queue games will I experience someone that is unhappy and make the rest of my game experience absolutely horrible. So much immaturity happens about just getting the role that you want that people forget that this is suppose to be a team game.
Why team builder is important is because when it comes to matching strangers with other strangers you want to reduce as much friction between teammates as possible. When it comes to strangers you can not expect people to go long with a completely new strategy when all the teammates don't even voice chat.
This feature does a great job of making sure a team of strangers are following the basic meta of the game. If you want to try out a new strategy or meta by all means try it just don't do it in a group of people you don't know. If you don't have friends to try a new strategy then use a regular strategy and help your team win and make friends.
Though there still will be many problems with playing with strangers that team builder does not solve. Players with low skill will still be matched with players of high skill creating friction. Myself included have let down my teammates and caused much anger to them even though I was telling them I was trying my best and just having a bad game.
The root of the problem lies with the internet itself and it's emotional gut reaction. Still to reduce as many misunderstandings as possible makes for a better online game experience. You won't be able to be able to get rid of all the anger, but with good matchmaking and changes to how pick-up teams are built you can make sure it is kept to a minimal.
Personally I don't mind low skill players that cost us the team the game since many times that will be myself. I play to have fun, but I still have a pet peeve in League of Legends and that is people who don't work as a team. With Team Builder I can at least be rest assured that we are all on the same page when the game starts. I now just have to worry about that skilled player that always try to take on the enemy team alone.