Feedback is the most important utility a designer can collect. Feedback can be both positive and negative but they are both equally important. Positive feedback shows the designer what is positive about their work and what they should carry on implementing throughout their designs and negative feedback shows what they should be avoiding or possibly change.
Feedback can be done personally, you can look through your own work and identify the pros and cons. But the most important way of gaining feedback is through a 3rd party, they look at a piece of work for the first time so they can identify immediately what is good and bad.