INI's are someone stored in cache (ram, temporary memory, whatever you're farmiliar with). As a result, when you use a command like /readini, you may infact be pulling a value from ram, not from the file itself. /saveini and /loadini alike turn virtual memory into file storage, and vise versa.
As with your question before, mirc does use ini's as part of the 'cleanup' process, not during runtime. But scripts you write do read ini's at run time, thus making the usage of /saveini and /loadini available.