if it isn't an eating contest (which are brand sponsored, it is all about advertising) it is usually at come-on, part of a restaurant's advertising to set them a little bit apart.
Saylor's in Portland OR has been offering theirs since 1946.
http://www.saylers.com/steak-challenge.html
If it were easy to be thin everyone who wanted to be, would be. The rest is all in fun.