HTML(HyperText Markup Lanuage) is the standard markup language to create web page while Mobi is a file format created by Mobipocket but now also adopted by Amazon Kindle Readers. You may need to convert HTML to Mobi when you want to read files saved from wabpage on your kindle devices just like reading other ebooks instead of reading them in the browser. Or you have written your own ebooks with HTML format and want to share them with your family and friends who own the popular kindle devices.