Why would you Parboil Some Vegetables Before Stir-Frying them?
Really, this is just to even out cooking times for vegetables that don't have a surface area to volume ratio consistent with the other things you're stir-frying.  If you were to shred those green beans, as is sometimes done, you could put them in at the same time as raw, julienned carrots, and they would finish at the same time.

If you put them in whole, you'll end up with beans that are burned on the outside and raw on the inside, as there's not enough time for the heat to penetrate to the center of the bean before the outside burns.  Or, if you fry at a lower heat, you could get the beans cooked, but would have to add the carrots later (which means a lower temperature, too, since the beans are dragging heat out of your pan) in order to have them not turn to mush.