When you have a web site, then you usually have complete control over it. You can put up any content you like and make the thing look exactly the way you want.
If you use the services of a blog host, then usually you have to conform to the general style of the blog engine. Your design options are limited and also the content allowed on the page is what the host allows - the purpose of the service is to host blogs (=text), not provide some web experience.