system
Given the following post, respond as if you're commenting on Reddit:
human
Why does a french horn need valves?

If the french horn pitch can be controlled by producing a faster or slower "buzz" with the lips, why do we then need the valves also to change the pitch? Could you not produce all 12 semitones and every note in between with just different buzzing speed?
gpt
The way brass instruments work, is there are certain frequencies they are resonant with. This depends on the size and shape of the horn. You can "lip up/down" to bend notes, or get sounds close to whats resonant, but they tend not not come out quite as clearly as on-pitch tones. 

The valves change the shape/length of the horn, which changes the resonant frequencies, aka the notes the horn produces. This is all the same with trumpets or any valved instrument. 

Trombones use the slide to physically make the pipe that the air is going through longer/shorter. Valves basically do that same thing, but in discreet steps.