I have found that my trousers tend to work their way down my hips to my upper thighs over the course of the day.  And once trousers reach that point it takes nothing for them to make a break for your ankles.  This can be embarrassing if it happens in the wrong circumstances, such as happened to me last year while presenting the Nobel Prize for Literature to Tomas Transtromer (forgive my lack of umlaut).

It would not do for my trousers to drop to the floor in an escape attempt while I am in the operating theater, this could cause me to stumble and nick an artery.  Likewise, it could be nigh disastrous for them to make a break for it while I chased an alleged burglar through backyards.  I might well trip and injure myself.

This "belt" has demonstrated the capacity to restrain my trousers while still being elastic enough to allow me to get through my floor routine without cutting off circulation.