Of course, a lot of us just find it great fun. Debian is so committed to free software that we thought it would be useful if it was formalized in a document of some sort. When you download a package from the Debian main distribution, you can be sure it meets our Free Software Guidelines. Although Debian believes in free software, there are cases where people want or need to put proprietary software on their machine. It is important to distinguish commercial software from proprietary software.
Proprietary software is non-free software, while commercial software is software sold for money.
Debian permits commercial software to be a part of the main distribution, but not proprietary software. The best way to learn about almost any computer program is at your computer. Most people find that reading a book without using the program isn't beneficial. Debian isn't as intuitively obvious as some other operating systems. Thus, you will probably end up reading at least the first few chapters.