I believe it helps with the way BBS loads long pages. Because they are only loaded a screen at a time, ctrl+f wouldn’t be able to search-in-page comments that are lower down. The BBS built in searcher will do though, if your’re searching within a thread.
This only occurs when the topic is large enough that it is not all held in browser memory. As you scroll through a large topic, just the visible section of the stream is loaded in the browser.