The odd thing is that, in the US, I rarely see anyone put any money into the tip jar. Most people I know seem to think this is tipping gone overboard.
I always assumed they got most of their tips from men who didn't want to deal with the change from their order.
For what it's worth, most of the tipping guides say there's no obligation to put any money in the jar, though some suggest it if they've done something special for you (above and beyond their regular duties).
I think this is from a guide for foreign students. http://www.eslc.msstate.edu/PDF/SOP_Tipping_Customs.pdf
Tipping IS very important in the US because many people in the hospitality industry (restaurants, hotels, casinos, etc.) are paid less than minimum wage because they are expected to make tips. Many people in these jobs rely on tips as a major part of their income. Not tipping—especially for a waiter in a restaurant—is considered very rude
Sometimes I see tip jars at fast-food restaurants, ice-cream parlors, or coffee shops.
Is tipping necessary at these places?”
No, tipping is not necessary in these cases. Usually tipping is required when people are actively serving you. For example, is a waiter is checking on your table often, then you should leave a tip. If a food vendor hands you a hot dog, then you do not tip
Of course, it may be different in Canada.