It's form the first episode of the eight season of Friends. Here is the context:
Phoebe: Calm down. Maybe you’re not pregnant. Rachel: What?! Phoebe: When I got pregnant with the triplets, I took that test like three times just to make sure. Monica: Yes! Maybe it’s a false positive. Are you sure you peed on the stick right? Rachel: How many ways are there to do that? Phoebe: I’m-I’m just saying, don’t freak out until you’re a hundred percent sure. Rachel: All right, I’ll-I’ll take it again when I get home. Monica: You-you gotta take it now. Come on, do it as a present to me. Rachel: Okay. Thank you. Monica: Okay. Phoebe: I’ll run out and get you one. Rachel: Oh, you guys are so great. Monica: Oh, wait a minute! Who’s is the father?! Phoebe: Oh no, she won’t tell us. Monica: Oh, come on it’s my wedding! That can be my present. Rachel: Wh—Hey, I just gave you peeing on a stick. Phoebe: See? This is why you register.
What does Poebe mean by this why you register?