With some functions, it's less obvious you really need to use the return value. Like string_replace_all(). When I was young, I spent ten minutes figuring out why string_replace_all wasn't doing anything. I was livid. It's a good case of RTFM, yes, but it's also a great case where we can save users a lot of frustration by using flags like need_result, or something.
This is basically a note for me to add the function into JDI, possibly with more compiler hints. If you have ideas for more compiler hints, post them here, so I can best think up a system for handling them.