why would you tip a barber? because they cut your hair good? what was the base payment for then?
Let me return the question: why would you tip a waiter? because they brought your food to your table? isn't that why they should be paid by the restaurant?
Let me also state that, residing in a European city, I do tip my (cheap) barber every time I go because I feel they do an outstanding job at 20% less the price than most competitors, and that work deserves praise in the form of a tip.
In general, it seems to me that a tip should reflect something outstanding. In addition to my cheap barber, I also usually tip at fancy restaurants where the service has proved very, very high above comparables.
Do you also tip your local bike shop or computer store when they sell you a computer or bike that is outstanding and costs 20% less than most competitors? Do you tip your car salesman? Do you tip your bus driver when he/she is 20% faster?
Tipping in the example circumstances I gave isn't typical, but it's something I would consider doing in exceptional circumstances.
Whether rational or not, I'm personally fine tipping where the worker is making a low wage, and have spent time personally attending to me. Part of it is thinking that they'll appreciate it more than I will miss it.