My siblings have played an exceptionally vital part in my life and I know I wouldn't be the person I am now without them. I can't confidently say that having a sibling will guarantee a positive change, but I can say that growing up with a sibling is an important and completely unique experience.
That experience is a child's first social interactions with a peer or at least someone who isn't their parent. They can play together with similar attitudes, learn from each other, and garner experience for socializing with peers that aren't tied by family. While this experience can be gained from a very early friendship, the odds of such a friendship happening or lasting in the way sibling relationships are forced to is unlikely.
Siblings can also have strong bonds that allow them to communicate and open up about things that they might be unwilling to talk about to their parents. I can certainly say that my siblings have helped me with a good number of problems that I didn't feel like I could talk to my parents about.
Those close bonds are also what makes a home truly home. Home is made up of people, and having close ties with siblings makes a home that much more like home.
I cannot attest for everyone's experience with siblings, but I have become really close to mine. My brother is someone I can always relax around and feel truly at ease. My sister is someone I can talk to about anything and she will always have some answer that can help me carry on.
I believe everyone should have the chance to have siblings that they can have their own tight relationships with. I can't imagine how I would have gotten here without their help, and I don't want to be one of a few who has that help.