1. People will find you and be informed about what you want them to know. Your blog allows you to update frequently and with ease, which creates ever increasing amounts of new pages for your website.. Google loves the kind of regular content updates that blogs produce, and indexes them over time, so that your internet presence and online reputation are built up over time, and people can find you when they are searching.
2. Use your blog to market your business. Include all sorts of information about your business on your blog, adding in photos, videos or others ways to display examples of your work. You should also list your products or services, your ebooks, articles, media appearances, or speaking engagements, or anything else that might help to market your business.
3. Blogging helps you to connect. Being a part of the world of blogging, means that you can network like never before. You can find others who think like you do, and become part of online communities all over the world.
4. Build your audience by blogging. Not only do you want to be found in the first place, but you want to keep them coming back for more. When people "subscribe" to your blog, they then become part of your following. By having a regular audience of "followers" that read your work, you can build relationships that will give you traction in your intended marketplace.
5. Learn how to leverage Web 2.0 on your blog. Two way interaction and feedback, are very desireable for doing business online. So allowing comments on your blog, does a lot to encourage direct connections with your audience. You should also do the same thing in reverse, by leaving comments on other blogs. This allows a level of interaction that can't be achieved by either email or static websites. An even newer  way to interact is to use those "Like" or "Tweet" buttons that are starting to appear all over the place. This takes the whole conversation to a new level. But, seriously, don't just use those buttons without also leaving a comment. It means much more that way. And it will come back around to benefit you in the long run.

These are just a few of the reasons why you need to blog , if you want to succeed online. There will be many more reasons to come...