One of my favorite things is the feeling of a man getting hard inside his jeans. A guy getting hard not inside his jeans is pretty awesome too, but there's something particularly naughty about the half-concealed boner. I can tell it's there, but he still has some secrets. I can squeeze it, but not stroke it.
I think it's like the difference between a naked breast and a nipple barely peeking through clothing. Being naked, that's utilitarian, that's how sex normally is. Being sexy in your clothing, the clothes trying to hide it but failing, that's a whole lot more interesting.
Plus, it's hard to get a guy's dick all the way out at a movie theater without someone noticing and getting all weird about it.