Why do witches fly on broom?

Answer 1

According to J.K. Rowling, brooms were chosen as the objects to enchant to fly because they were easy to hide.

Fictional witches ride brooms, real witches ride more mundane things, like cars, trucks, motorcycles, horses and bicycles.

Answer 2

Hopefully you're over eighteen, because the answer involves mind-expanding drugs and genitals. In medieval times, 'party drugs' were on festive occasions regularly taken for the same reason that partygoers today use Extacy pills: to be able to keep on going for longer, shed inhibitions and generally, to feel very good about themselves. The concoctions that were made up at the time (nicknamed "witches' brews") were found to be more effective and to last longer if absorbed through the skin instead of being swallowed.

What you were looking for then, was a place on (or inside) the body where the skin was thinnest and would most easily absorb the active ingredients of the herbs - and where the herbs would remain in place even while the partygoer was jumping around on the dance floor. That's where a man's single and a woman's twin orifices came in. A smooth stick which might be a broom stick (a ''witches' stick", indeed) was used to push the herbs in place which for some may have added to the fun. A common effect of the witches' brews was that the user felt the sensation of flying high above the earth. So that's how witches, broomsticks and flying around got connected.

So next Halloween, if you get someone at the door dressed as a witch and carrying a broomstick, make sure what he or she intends to do with it if you don't deliver the candy.