Not sure if it's an appropriate question. But I just can't help wondering, why would any Vim user find it necessary to invoke a GUI search prompt instead of search using / or ?, why would the designer of gvim, MacVim etc. want to add this feature. Or is it because they're just quite some inherent feature of a GUI app anyways?
What's the point of :promptfind after all
gvim;search;macvim
:promptfind and related commands are useful for the easy mode (evim), which according to :help easy sets Vim up to work like most click-and-type editors. As Notepad has such a find dialog, and users coming from such editors expect it / initially feel better with it, Vim has it, too. I personally wouldn't recommend this way for switching to Vim, though.