That is explained in a previous post.
Not really why the search history list is insufficient though. The OP gives an anti-use-case. I would state the usefulness for this enhancement more positively as:
It's convenient to leave the find dialog open since it's non-modal (similar concept of popular find bars in-window). When you are about to search for something, it's helpful when it can automatically be the term you're focussed on/cursor at/selected (Geany even does it for you if you press Esc to hide the dialog before performing next search). Sometimes I want to re-find the same thing as I did last time when I press Ctrl+F, so it's helpful that Geany keeps a search history right in the search text field so after I press Ctrl+F, I can use the arrow keys to navigate through my previous searches, causing only one-keystroke to bring up my last search. Geany is smart enough to only add words I actually searched for to the history so it doesn't get clogged with stuff I never wanted to search for that it filled in automatically.
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