They can be an invention, a product, social change, just one or two of each, but what for you is usefull and useless and why?
useless...the scart lead.
I want to meet the twat who invented this piece of crap, not for the idea of a cable that can carry more pieces of information, but for a fitting on either end that falls out of the appliance if you so much as sneeze near it, fucking useless without a doubt.
usefull...the toilet duck pad cleaner.
This little plastic wand has a trigger on it, that opens and shuts some jaws at it's end, into which you insert a small cleaning pad which you then release into the toilet bowl once you've cleaned it. A very real and hygenic replacement for the disgusting Hedgehog on a stick.