A writer needs them. Possibly more than anything else. Without friends, a writer is in trouble, in more ways than one. A friendly eye on a manuscript is invaluable, whether that friend is a family member, your best friend, or a fellow writer you met on a writer’s forum. It doesn’t matter.
No writer can write a perfect book alone. Even the professionals, the ones who turn out several books a year in return for crazy amounts of the papery green stuff, have editors. They’re usually friends with those editors, for obvious reasons. If you don’t like your editor and the feeling is mutual, you’re going to have a hard time getting much of anywhere.
A friend will read your manuscript, that slab of wood pulp into which you poured so much time. They aren’t critics; they’ll do their best to leave your pride intact. But a friend, providing they’re the real thing, will make sure you know all about the mistakes. They won’t let you toss your book out into the world if it isn’t ready. The authors who can edit their own books to perfection by themselves are few and far between, possibly even more rare than unicorn’s teeth and hen’s mane.
The perfect author doesn’t need friends, he just needs critics, people who can be amazed at his work. Critics are there to criticize, nothing else. You can impress them or disappoint them, but that’s all. Friends, editors or not, don’t criticize, they point out faults. They don’t publish scathing reviews, they help protect you from them. They don’t make sarcastic similes, they fix cliched plots. Friends don’t crush your dreams, they repair bad foundations.
Without friends, a writer is in deep trouble. It’s hard to give a complete stranger your manuscript to edit. Even when you do, their opinion is suspect, mostly because you don’t trust a stranger. In the course of the reading, you can come to trust that person, but then they’re no longer a stranger, they’re a friend. Or they might show a critical, but unhelpful side, and reveal themselves as a critic.
The best friends for a writer are other writers. Preferably ones who are good at what they do and who, if not actually writing in the same genre as you, enjoy reading that genre. The elements of good writing are universal, regardless of the genre. (non-fiction can have different rules than fiction, though) I have several friends who are writers, one of whom is published and one of whom is a remarkably perspicacious amateur. With their help (and the helpful, if caustic advice, of my sisters and mother) my writing improves every time I sit down to work.
Without friends, an author’s writing would improve on a painfully slow basis, if it improved at all. The improvement would rely on the author being able to see his own faults, which is a talent very few people have in great amounts. I don’t have it, or if I do, it hasn’t started working yet.
Get your friends to read your work. You’ll get more out of it than they will. If you don’t have friends, you’d better make some fast.