For some, the answer is that they have something to say and need to get it off their chest. However, in the context of your business you need to think about a blog as a demonstration of your expertise and a way of engaging with potential customers.
Blogs should provide useful information, but not so much your services aren’t required! You are looking to enhance your business not to make it unnecessary for your customers.
Blogs are also useful for expanding the knowledge of your clients so they understand what makes your particular business of value to them. An educated customer will have more appreciation for what you do, than a customer who does not understand the full spectrum and value of what you can offer them.
The final point about blogs is that they are relatively easy to do. You don’t have to be a computer geek to add posts to your blog. Furthermore, since search engines (like Google) adore content, the more you blog, the higher your website ranks in searches. This will help you get more clients for your business. A word of advice, if you commit to a blog, stick with it and blog regularly, it will pay off in the long run.