I can think of at least two reasons to care. One is just knowledge and understanding. The source files reveal the working of the operating system much better than anything can. The change history at the top of many of them start in 1980, with entries by Andy Hertzfeld or Bud Tribble. If you are a computer archeologist (aren't we?), these files are your Rosetta stone.

The second reason is the one mentioned by Dennis Nedry in his original posting. If we have the code, we could each make our own customized version of system 7, that supports the equipment we want to support or adding features system 7 never had.