Why would I tip someone who makes really good money to begin with?

I was reading an article about tipping that suggested tipping garbage men and postal workers. Both groups already make pretty good cash, and I never see any of them in private, so why would I give them a tip? It'd be like tipping an electrician who I'm already overpaying to work in my house. Are things just different between major cities and rural areas? Because I live out in a rural area and all of that just seems odd to me.