Can anyone explain to me the importance of module scripts?

I really want to know the importance of module scripts and why people use them.

I don't use module scripts very often but they're for storing functions that may be used by another script. Say you want to conserve space on one script, so you put it into a module script.

This basically means just calling functions from another script to save space or you'll be confused with all this code that you have to search through.