Why do we use the bathroom?

It depends on what আপনি are asking.

Why do we use the bathroom for pooping?

Because it would stink if someone took a shit in your bed.

Um, because...

I know why, but it's kind of long to type and I'm not your science teacher.

Because we don't want to go in our pants now do we? :)

so i can pee

hello! so we don't use diapers XD

Because we are civilized, dammit. Here's an interesting factoid I learned on the History Channel: Back in the days of Marie Antoinette, the palaces didn't have bathrooms, so everyone, royalty included, did their business wherever. It's something they don't tell আপনি in the textbooks.