It's important because if you are only looking at things from your own point of view you run the risk of tunnel vision.

Input from others widens your vision to include what they also see.

To become a more indistinguishable part of the masses?
;-)

Because if we are egotistical enough, we can end up overrating ourselves. "I wish someone has said that to me long ago" is a something we say to ourselves. There are too many "yes" people.

To calibrate where you are in relation to that person.