Today's amps switch clean and distortion with a switch. An obvious function. With one footswitch, you cross between a clear sound and a fierce one.
But this "obvious" wasn't there from the start. The idea of covering multiple sounds with one amp was born, and settled, at a certain time. I want to follow when, and why, it became so.
Once, an amp was one sound
Early amps had no concept of "channel" in the current sense.
Even with multiple input jacks, that was a difference of input, not a switch of voice. One amp was basically a device that makes one sound. Want distortion, raise the volume; want to return to clean, roll back the guitar's volume. Switching was covered by the player's operation.
To change the sound, you had to swap the amp itself or improvise at hand. One amp = one sound was the premise.
The circumstances that demanded "multiple sounds in one box"
Eventually, the demand to make multiple sounds in one box grew strong.
Music diversified, and scenes of crossing between clean and distortion within one song increased. Lining up several amps on stage isn't realistic. If so, you want to hold several clearly different sounds inside one amp. This demand pushed out the channel design.
Prepare separate amplification paths for clean and for distortion, and switch by a switch. Make several characters coexist in one box. Channel switching was a response to this demand of "make one box be enough."
Switching is also about reproducibility
The settling of channels has another meaning.
In the era of changing the sound by hand, it was swayed by the day's condition and quirks. Fix the sound as a channel and, whenever you switch, the same clean and the same distortion come out. Switching is a mechanism not only for convenience but for pinning the sound down and making it repeatable.
This is, you could say, the first step of the flow leading to presets and profiles. The shift from "making the sound on the spot" to "preparing it in advance and recalling it." Multi-featuring increased reproducibility, not hesitation.
Each channel has a different sound
Here, put a recording viewpoint.
That there are multiple channels means there are multiple different sounds inside one amp. Even the same model, without saying which channel, at which setting, you can't pin down the sound. Say "that amp's sound" in one breath, and inside, several characters coexist.
So to keep an amp's sound, one per amp is sometimes not enough. Including which channel, in what state, does it first become one sound.
In short
Channel switching wasn't a function there from the start. The demand to cover multiple sounds with one box, and the demand to fix the sound and repeat it, settled this design.
It was the beginning of the shift from "making the sound on the spot" to "preparing and recalling it." When discussing one amp's sound, "which amp" alone is no longer enough. Only by saying "which channel, at which setting" can you pin down the sound.