Why do people call/re-raise preflop?

I have only experienced this a few times so far in my young poker career, and my first instinct is that it's a completely silly play. Since I've experienced it, I've noticed it come up in a few of the hands posted in here. I just can't understand why a thinking poker player would ever do this. I couldn't find anything about it in SSHE.

Can someone please enlighten me as to if this is ever actually +EV? Is there ever a situation where this play makes sense or is it pretty much just a boneheaded move?