The author probably cares about either copy-left or the right to tinker. Some people believe it's important and others don't.
Basically any derivative product out of GPL3 code needs to either be open-source/copy-left, or if it uses the code as a library needs to let end users substitute that library for their own version.
Basically any derivative product out of GPL3 code needs to either be open-source/copy-left, or if it uses the code as a library needs to let end users substitute that library for their own version.