In a word?
* Time to write. -- Away from your family, friends, job, house, chores, farm, pet dinosaur, and anything else that interferes with your writing time.
* Little or no responsibility for meal preparations -- As a mother of five children, let me just say how awesome it is to have people make food and feed me. Not mac and cheese either. We're talking crepes, Hawaiian haystacks, salads, fruit, roasted chicken. Heavenly.
* New friends! -- even the most introverted of writers comes away from retreats with a few new friends. Writers usually get along famously when tossed in a room together. Don't be surprised if you hear snippets of conversation like, "Would a body be harder to find in a lake or in the ocean?" Plotting murders make for great bonding time.
* Free help. -- With a house full of writers, it's no biggie if you get to the end of your novel and can't figure out how Joe rescues Fifi from the White Lord, because you've got a captive audience that loves to brainstorm! With all these brilliant minds bouncing ideas off each other, there's bound to be something that sparks the idea to get you over the block. (this feature is also extremely helpful for spelling and punctuation advice. Don't fear the Grammar Nazi's, they are harmless as long as the chocolate doesn't run out.)
* Nice rooms -- Most retreats offer nice accommodations, though you might be expected to share a bed. If that is a deal breaker for you, consider bringing your own cot and/or mattress.
* Bonus Prizes! -- At many retreats, there are door prizes or swag and awards for the highest word count, early birds, night owls, best quotes, or most pages edited.
* Classes and/or Workshops. -- Sometimes they offer classes on marketing, brainstorming, or any number of other subjects. They can even watch and analyze movies and discuss plot points as they come up.
As I write, it's the last night of the iWrite retreat in Heber, Utah, and we're all up late to savor the last of our freedom here. I won the award for being most cheerful (I think it's probably my funny writer shirts). It might be silly, but I almost teared up that people enjoyed being with me as much as I loved being with all of them. Alas, it's time to go, but I'm already looking forward to the next one in the fall.
If you can swing it, GO. Retreats are wonderful!