So that you know it worked. Even without looking at the small display that also usually changes in response.

This is what I miss about older technology - especially dumbphones. The input always worked, and always worked with the same timing. I could operate my Sony Ericsson K800i without looking at the screen because my brain memorized the sequences of buttons and timings of common operations. Joystick press, up, up, press, down, press, wait about a second, press, done. Compare with Android phones which routinely ignore or misinterpret input ("that was a swipe, not a press!"), and hang at unpredictable moments for unpredictable amount of time (usually also ignoring input while hung).

One benefit, though not necessarily intended by the manufacturers, is that blind users get some kind of feedback for each keypress, since there's usually no tactile feedback on a microwave.

I once spent half an hour reading the microwave manual (in Japanese) looking for some hidden way to disable the beeps. If you hold down one of the keys for over 5 seconds it puts you in silent mode, also disabling the annoying melody when heating is finished.