There are many reasons you should have a blog.
I am going to focus on it from a business and promotion angle.
When I create a website for someone, they are paying me to
design and set it up and then I get an hourly fee to update it and
do the usual things a webmaster does.

Many websites are simply used like an office address.
An address online and a place to find the business and find the person.
Here is where you find their contact information or speaking and event calendar
and any products they may be selling.
This is critical for anyone who is promoting themselves or their product.
Everyone needs a website. The Internet is today’s yellow pages.
When you add a blog, you actually end up with two websites for the price of one.
You also can run a blog yourself apart from your webmaster!
This is fun, easy and critical in keeping your
website and name fresh and alive on the internet.
How does it do that?
Search engines love fresh content.
Most websites tend to be static.
After the initial launch they die in the rankings and the information you have becomes stale
and lands on the bottom of the heap when someone does a search.
Example- if you wrote an article on “How to Create an Ebook” and had it on your website,
it would get indexed in the search engines and might pull up if the keywords
were good and you had a good helpful article.
If you wanted to keep getting fresh traffic and good rankings,
every time you wrote a blog post with anything to do with your topic you
would add a link back to that article, and eventually this grows over time,
and when blogging, the feeds are picking up your topics and highlighting
and distributing them and then there are things happening with your posts that
require no effort on your part, other then to keep posting fresh content on your blog.
You can either get a free blog, here http://wordpress.com/ or http://blogger.com
a google company,
If you already have your own website you can install
or have your webmaster install one of the themes you choose from here http://themes.wordpress.net/
Then you will get a login and password. Be sure to login and take a look around
(you do need to familiarize yourself with the layout and how to post)
Spend some time figuring it out, it isn’t hard and all sites have excellent forums and support if you need help.
Don’t add a blog to your site if you’re not willing to consistently some invest time and effort.
A blog with year old posts or months that have passed will work against the
fresh imagine you want to convey.
They also look abandoned.
I tell people to try to post at least once a month.
I think that is doable for anyone.
Weekly is better and daily is great if you enjoy it.
Most people and companies I know do not post daily.
That is ok, as long as you make sure to keep it up at least once a month.
Every time you create a new post/article/entry
it is picked up by various search engines and feeds.
Over time this will help propagate your site and blog.
Feeling overwhelmed by the idea of trying to write something on a regular basis?
Ask people you know to write a post for you and publish it
This is a great way to network and people feel privileged to have their article on your blog.
You also can get articles from the many article websites online, like e-zine which allow you to reprint and publish articles.
Once you add your blog, add a google alert http://www.google.com/alerts
(you can sign up for these and google will email you when other sites
are adding your site or giving attention to your website or blog)
to see what feeds and other sites are noticing your blog,
and also be sure to start checking your traffic stats, so you can see the increase in visitors to your blog and website.
Once I added a blog to my ebook website, I saw my traffic go up by 400-600 visits a day.
Now that is good value for just spending a little time and effort maintaining a blog site.
Have fun choosing your theme and I look forward to finding your point of view
and whatever you have to offer the world on your website and blog!