The most important part: much less eye watering.
And then do in reverse once it's clean. And you're wondering why it seems like too much trouble...?
Plus you still need the knife and cutting board anyways to chop off the ends of the onion before peeling it. So it's not even instead of, it's in addition to.
Much less time to dice it yourself for one or two onions. Ten or twenty onions, OK it's food processor time.
Overall, doing all this is much more convenient than manually chopping onions. It's not even close. This is one of the ideal use cases for a food processor.
If it's really bothering you, make sure your knife is sharp enough. A dull knife makes dicing much slower and releases many, many more compounds.
But I don't really have trouble with my eyes with onions, that may be the deciding factor.
I stick my food processor blades in the dishwasher all the time; never hurt them any.