I have not seen these jars "everywhere" I go. I have seen them in a couple places where the service is designed to customize something for the customer - like my local Subway. I also see these at Mongolian barbecue inside Chinese Buffets (or a exclusive Mongolian Barbecue restaurant).
For the Mongolian - the tip jar is there to tip the chef cooking your meal, where like at some of those Japanese restaurants that cook the food at your table, they put on a little show while you watch your food cook, so that tip is (1) customary and (2) like giving money to street performers when you appreciate the performance.
I do occasionally tip at Subway - typically it's when the staff goes above an beyond. I'm a "regular" there, so when an employee remembers my entire order without input from me - I provide a tip for that above and beyond service. I don't always tip, but occasionally when they impress me, I appreciate they allow me to tip.
I've actually seen more charity jars for people to place change into than tip jars, and actually see places that have "no tipping" signs.
But that's just my experience.