Because another set of eyes is invaluable. When you write, you know the story and your characters intimately. Your mind fills in the missing pieces without even realizing it. You are too close to things to see them clearly.
We do our best to articulate what's inside our heads, but we don't always see the varied ways it can be perceived.
If you have others critique your work, you can see what makes sense to the reader and what doesn't. Patterns might be obvious to someone else. You will learn your weaknesses and by doing the same for others, you learn as well. It's a win win situation.
But there's more! You also learn to take critique. It isn't always easy to hear anything other than, "This is fabulous." But you need to be able to consider the critique objectively if you want to get anywhere as a writer. What do you think your agent and editor will ultimately do? It's good practice.
And by the way, even positive feedback is essential. You need to recognize your weaknesses and work them, but you also need to understand and explore your strengths. Take pride in them too, and don't gloss over the positive comments.