On several occasions I've seen these things placed in the seat portion of the grocery cart. It seems people these days are becoming more and more germophobic. Seriously, they put these on their carts to keep their precious little spawn away from any and all germs.
What they're really setting their child up for is being seriously ill later on. A child who is not exposed to at least some illness-causing agents will not be able to build any immunity. Using something like this really is a disservice to your child1,2.
My belief? These parents fall into one of 2 categories: Those who are true germophobes who's cleaning-product shelf looks that of a hospital ward in charge of taking care of immunocompromised patients, and those who just simply don't want a sick, whiny kid with a runny nose, they also have similar shelves in their homes, sanitizer at every sink, and will generally sneer at you, should you get too near their precious child.
I don't quite understand this culture of needing to raise your child in a perfectly sanitized pristine environment. And FYI, those covers are likely to be even more of a breeding ground for dangerous bacteria than the smooth plastic and metal of a shopping cart.