One of the benefits of a WordPress site is that you don't need to start from scratch if you have already created a site using a similar service somewhere else. If you have previously used any sort of "blogging" service such as MoveableType, TypePad, LiveJournal, or Blogger then you can use the Import facility within WordPress to import your posts from that blog into WordPress. As WordPress actually provides the import functionality for these alternative blogs those blogs presumably have the appropriate export options available in order to produce the files in the format that the importer expects to read.
The import option also makes it easy to transfer yo9ur data from one WordPress site to another by using the export option to first export the data from the one WordPress site and then using the WordPress importer to import it into the second site. This option is particularly useful if you have been using a WordPress.com site for some time and have now decided to switch to having your own hosting. This also provides you with one alternative for how to transfer your WordPress site from one hosting to another.
The site you import doesn't even need to be a blog. Any type of site that produces an RSS feed of the site content can be imported into WordPress using the RSS importer.