Access level 'system': what does this really mean?

The OLFM User Guide defines it as "field values that you cannot add or delete".  Then what is the Enabled check box for or the end date?  I am referring specifically to OKL_REBOOK_REASON, but the same questions apply to any lookup with access level = system.  If you disable them, or end-date them, which of course is different than "deleting", aren't you simply preventing the user from selecting that code and therefore the code in the form corresponding to the disabled code won't execute anyway?  The same thing with adding a code - if there is no requirement for the