A logo is about branding your business. What do you see in your mind when you think of Apple, Nike, Coca-Cola, Google and even Etsy. They all have strong logos that conjure a mental picture of the brand. You can use it for everything...business cards, facebook page, advertising sheets, enclosure cards...anything you do can incorporate your logo.
I also appreciate having it as a starting point for all of the graphics associated with my shop. It makes it so much easier than starting from scratch when I'm laying out advertising pieces, especially for print advertising.