If you know how to blog from your own website then you will have many options open to you in the design and functionality of your blog that are not available on a hosted blog.
You can use Blogger from blogger.com or WordPress from wordpress.com. Each has its own peculiarities, benefits and disadvantages, such as the lack of trackback and categories on Blogger, and the relatively poor raw html editing facility on WordPress. However, each has its fans and adherents, and those used to one or the other are generally loathe to changing. Neither, however, offers the wide range of options open to those that have chosen to run WordPress or any other blogging software from their own webspace.
Those who use the hosted versions likely do so either because they have no need of the options that the uploaded software offers, and are happy operating a blog using the range of options offered by the blog host. Others might not have a website, and have no intentions of having one because they have no need for their own site. Many bloggers are not involved in internet marketing, and are not using their blogs to make money not even Adsense revenue.
They enjoy blogging about...