For the sake of comedy, I'll answer this one seriously. From what I understand, guys usually tuck up their "junk" in order to hide erections. Because they can occur at random they always have to be prepared. If an unexpected erection occurs and the "junk" is just hanging there, it will get stuck awkwardly in the underpants. If it is pointed up, it will be kept flat against the body by the elastic band in the underpants. 

Content with this answer?