When you're writing you have a very clear idea of what the characters are like, from their attitudes and motives to what they look like. You can see quite clearly the places they visit, the roads they drive down. This one is charming, that one is self-deprecating, the other is witty. What a shock it is to discover that readers find the charmer a pain in the neck, the diffident one aloof and stuck up and the witty one a complete pillock. No, no, no, you cry. That's not what I meant at all.
It happens a lot in class, particularly with people who are new to the workshopping process. They like to explain what they meant by their writing. This is fine for class, but it won't work when they send their writing out. Afterall, they can't tag along, making sure the reader gets that that comment was meant to be ironic, not taken seriously, or explain the reason they suddenly turned left - it was the complicated one-way system.
More experienced workshoppers listen quietly making notes. They know what they meant by their words; they want to make sure that the readers understood that. Recently someone wrote something so subtly that everyone was baffled by what had actually happened. All it needed was a few extra words and all became clear.
Feedback gives you access to what readers get from your work, and the chance to make sure it tallies with what you want them to get. No one need ever know if you take feedback on board, there is no obligation to make changes, but you should at least listen to what people say. I'm a big fan of feedback and don't think I'd be published without it.