1. Disagreement can result in even newer, better ideas. Debate is healthy for society. If people always agreed, they would never feel pressured to try new ideas. Disagreement can spur discussion, and sometimes the outcome of those discussions can result in a new idea that is even better than the original topic.
2. Education: It's good to see someone else's perspective, even if you do not agree with their viewpoint. Learning to be flexible in your perception will make you a valuable team player and group member.
3. Life cannot always be 'Burger King' where 'it's your way, right away.' Learning early on to deal with disagreement teaches you how to overcome obstacles, but also how to deal with being told 'no' sometimes. Disagreement can strengthen character and resolve.