For years now it has been the custom to tip, tip waiters/waitresses, taxi drivers etc. It has become the "norm" as most people, when mostly eating out, think tipping is obligatory, either that or they don't want to appear mean so feel obliged to thank people by tipping, I count myself included in this over the years. In some cases people have left a [good] tip only to find out when they check their bill a service charge was added to the bottom of the bill anyway meaning a bill that should have cost you £50 has now cost you well in excess of £60.
This can have a negative impact, for example at a restaurant if you have a meal that's not up to par or bad service {or both} most people will still feel obliged to leave a tip and will just not go there again instead of showing your displeasure by not leaving a tip at all, this is why people put their experiences down on Tripadviser warning others you have had a bad meal by putting a review saying it wasn't worth the money but you paid over the top for an the experience.
A few years ago I pointed this out to a friend of mine who had come back off holiday, he was quite happy to tip when eating and drinking and taxiing here there and everywhere whilst over there, yet over here his tipping is limited only when dining out or using taxis. He never thought to occasionally tip the person behind the bar of his local who probably serves him every time he goes in or saying to the bloke in his local chippie "keep the change" on his way out, yet has no qualms about tipping total strangers who he may never see again.