I believe the tip jar at the coffee shop started when coffee shops, like Starbucks, started making more and fancier drinks. When I get a simple cup of plain black coffee I don't tip but when my wife and I are together, I get my black coffee and she usually gets a latte or whatever they call some of their more complicated drinks. I'll tip then because they actually put some effort into making it and I want them to remember us and make nice drinks for my wife the next time we're there.