Well, webpartzone, and webpartmanager are just classes of the .NET which you can utilize in any Asp.Net application, just like Sitefinity. The main purpose of Sitefinity is to deliver a platform for developers and set of features (Blogs, News etc) that can be customized to fit your needs. You can have many other features built-in, rather than just web parts.
As for the difference with Sharepoint, we are focused on easy content management and end-user friendliness. Some people even create their content in Sharepoint, but use Sitefinity for their front end.
the Telerik team