Why should your business have a blog?
I was hard pressed to come up with any better reasons than the ones listed in Lip-Sticking: 5 Reasons Jane Thinks Every Small Business Should Blog. If you have a business, and you don't have a blog, I'd recommend that you pay her site a visit.
As Yvonne Divita states, it's about improving communication with your customers. A blog can be very easy to set up, and if you like writing, can be a great way to get online quickly and easily.
I'll add to her other points that it can be fairly inexpensive to set up a blog.