When expressed in a civilized manner, disagreement is a beautiful thing. It shows that people have different ideas on a particular topic; difference means variety, and that people aren’t just going to kiss-ass and not think for themselves.
It also means that there’s an opportunity to come to an understanding of other viewpoints and to come to a compromise if necessary. ”Even though I disagree with what you say, you made some good points!”
…all of that is in theory. In practice, well…see any political discussion ever.