It polishes our writing until it is perfect for the market we are writing, or even just perfect for you.
You can't be afraid to delete what doesn't work, what takes too long to say even if you think it is your best work. This is ego. Many writing gurus have used the phrase "murder your darlings". These are the bits you are particularly proud of. It may be hard but it is worth it in the long run.
Guaranteed these "darlings" don't fit the writing and it will save time (and pain) when you are asked or requested to delete it from an editor or publisher, or even a reader may not get it.
By doing this we grow as writers. Editing helps to polish and give us the best our readers can get.
Rob Parnell just wrote an excellent article called How Does your Writing Grow? i would recommend you read it. But pertinent for editing is the part:
You have to make writing 'work'. And if you're reworking material, don't just fix things, rethink the whole piece. Take onboard the suggestions, yes, but also be mindful of the entire package. Do the suggestions change the way the writing works? Do you have to approach the manuscript again - and make it work, again?
If so, don't be afraid to do that.
My novel is currently undergoing a re-write. I am taking on board my readers advice and the advice i have received from Rob, and where sometimes it is hard to "murder my darlings" i can see how it works to make my writing work and make it better.