Writers are readers.
I have always been a reader, even when I hadn't written anything in years. Most of the writers I know not only read, but read hundreds of books a year.
I could understand if we were trying to sell waffle cones. Then that logic would make perfect sense. If I own my own waffle cone factory, then I don't really need anyone else's waffle cones. But who reads their own books for entertainment? Books are a case where suppliers and consumers overlap.
But that's not the only reason it's great to have writer friends. When you are stuck or going insane or feel unmotivated, your writer friends understand the exact hill you're trying to climb because they've had to climb it too.
They are founts of valuable information. Just reading my friends' blogs about what has worked for them and what doesn't work for them gives me all kinds of ideas about writing, marketing, etc. And most importantly...
Writers aren't in competition with each other. No one reads just one book. That means there is no reason that writers can't work together to promote each others' books. When I read a book I like I scream it from the rooftops. Why? Because if I enjoy a book, then people that like my writing will probably like the books I enjoy. That way people get extra value for listening to me blather on. I help them find things they like. And that makes me feel all warm inside.
So go hug a writer today.