This is like asking "Why does a kitchen stove have a range of settings on a dial when simply having the burner be
on or
off would also be able to cook food?"
And the answer is simple: Because they can be useful settings.
The food being warmed does conduct heat, but it is not an ideal conductor of heat.
And microwave ovens (just as any other kind of oven I can think of) heat from the outside. And they're supposed to make things easier and simpler.
So let's make an example: Leftover refried beans, still cold from the fridge.
I can put them in a bowl at 100% power for three minutes, and they'll probably explode and make a mess and still have parts that are cold. This "works" but it's obviously not very good. (I can mitigate some of the mess by using a cover of some kind, but that's also kind of shit.)
Or: I can put them in for a minute or two at 100%, and then stir them, and then run them for a another minute, and then stir them again, and maybe then do another minute. This "works" but it's enough work that perhaps I would be better off to skip the bowl and warm them up in a pan on the stove instead.
Or: I can set the microwave to (say) 40% duty cycle, and put them in for whatever I think is a reasonable time for the volume of beans at that duty cycle. Let's say 5 or 6 minutes.
It's slower, which prevents layers from getting stupid-hot and explodey, and gives the beans more time to reach thermal equilibrium. It's completely hands-off once the buttons are pushed. I'll probably still give them a stir before serving, but they'll be fine.
(I'm a fan of simplicity, but I'm not a fan of lack of control like OP's microwave offers as a primary selling point.
The microwave oven that we had when I was growing up was simple and functional: It had mechanical timer switch with a mechanical bell to set the cook time and announce the end of a run, which is about as simple as it can get while retaining any aspect of automation. It also had an analog dial with which the duty cycle could be continuously set, from somewhere between ~5% to 100%, and this duty cycle could even be changed while the machine was running. No computers, and nothing particularly electronic at all. Just a timer and [what was probably] a heated bimetallic switch (just like a common, cheap electric range uses).)