The packages in deb-src repositories contain the source code for binary packages in deb repositories.
These source packages are free and you can do what ever you like to do with them, provided you stay within the boundaries pointed out in GNU General Public License http://www.gnu.org/copyleft/gpl.html
For example, you can compile programs from source packages. Or you can modify them to your liking. Or you can fix bugs from them and submit patches to developers. Some hard-core geeks like nothing better than spending their evenings just reading source code. But you'll probably need to know the programming language they were written with to really appreciate the finer pleasures of reading source code.