That's so they can do stuff like better access your G-Spot. If his curved penis bends to the south, you guys should try out the doggy style sex.
But if you have a more significant bend in your penis, which may cause you pain or difficulty having sex, see your If you do get pain, it may get better over time.
Learn when a curved penis is a problem and when it might actually be an advantage in certain sexual positions. All your Are curved penises better for sex?.