The TeX Live installer gives the option, checked by default, to "install font/macro source tree". The documentation states simply that omitting these files is "not recommended", with no further explanation.
Presumably they are useful to have on hand for writing or modifying macro packages and LaTeX classes. What use, if any, is there to installing them if the user will never do anything other than using existing packages and so on to write documents? For example, are they ever required for debugging?