Due to the tight nature of this apparel, it is extremely difficult and awkward to conceal this kind of erection.
Not only is the fact that you are publicly 'excited' embarrassing, but, because the wearer should be fully aware of the nature of the obvious lump in his jeans, he will not want this erection, thus decreasing the size. This, however, is not the only factor in the embarrassment - the fact that the jeans are so tight also cuts of blood flow, further adding to the illusion that the wearer's member is smaller than it is.
Something to be avoided if it can be helped.
Girl: OMG! It's tiny!
Boy: Nah, it's just a drainpipe boner!