Horns didn't originally have any valves at all. Especially in Mozart's day. They were basically German (not French) hunting horns, adopted for orchestral use. Here's an example of the same Mozart concerto played on a natural (valveless) horn. Can you tell the difference with your eyes closed?
The first use of valves on a horn was designed just to add different slides to change the horn's key. It was a lot more convenient than pulling out one slide and replacing it with a longer or shorter one. (Often, concerts involve various pieces in different keys, so a quick change was an advantage.) The hornist still played different notes just with his embrochure and hand in the bell.
