Because they're funny on many levels.
You PO PO PO POKE HER FACE PO PO POKE HER FACE!
Me: My boss told me to go to hell
Just look for ones that have a "JOKE:" disclaimer
Guaca**mole**y edit: how do I make it not say this joke is Dirty
He was feeling down
It lifts their spirits.
Expectorant(/spoiler)
They work on so many levels.