why should you pee after sex?

i’m watching a show and a girl didn’t and she is in pain and mad she didn’t pee after sex. i’ve heard you’re supposed to but i have no idea why. i figured it is better to ask here than google it lol

thank you all for answering! i know it was probably an incredibly dumb question and i should be old enough to know that, but i come from a religious family so it’s not like i was educated past what condom is and how it needs to wait until marriage