You can learn what every process is there for, what implications removing it has, or which alternatives for it there are. For example, it's common to run a Unix machine with just a WindowManager instead of a full DesktopEnvironment, or even entirely without a GUI, if it doesn't need one, thus saving plenty of resources.
- It is easy to find out what any file is needed for. Easily viewed and edited plaintext files are common.
- Traditionally you get at least a large chunk, if not all of the SourceCode of your Unix system. You can always read it if in doubt. (And even lacking source, strace(1), and ltrace(1)? will both help you debug problems.)