Why do we need to wear underwear?

I sort of came across this when i was wearing one. In my opinion,
1. ) to avoid other people from noticing that change
2. ) cleanliness
3. ) avoid the balls from sagging
4. ) girls wear it to support the pad

Oh yeah, and why do girls need to wear underwear apart from what I have mentioned above? That only occurs like 5–7 days in a month so why there’s a need to wear it on other times? I figure that, there might be some leakage if they happen to be horny that will stain the pants. Is that a good answer?