It depends on the format of the content you want to include and your specific model of Kindle. In general, you can transfer
.mobi files directly to the Kindle using a USB cable. Put them in the
Documents folders on the Kindle (differs by your specific model) and they should appear automatically when you disconnect from the computer. There are step-by-step instructions here, though keep in mind that you will want to put ebooks in the Books folder on a Kindle Fire (and possibly other, newer Kindles), or else they will show up in the wrong place on the device.
If you have files of a different format, such as
.epub then you probably need to convert them. (PDF can be viewed directly on some Kindles, but it doesn't always display in a reading-friendly format, so conversion is advised.) Calibre is one tool that you could use to convert them to
.mobi or you can Google
"convert xyz into mobi" (where xyz is the particular file type you have) and you should find several sites and programs that can do the conversion.
As for the email address, that is a "backup measure" in that it is the simplest way to do it for most users, and Amazon does not target their devices only to the technically minded, so most users can just email what they want to see on their Kindle and it will show up.