The basic concept of tips makes sense: you are giving a small token gift for those who go above and beyond the basic service you would expect for the basic payment.
The problem is over time so many people start doing it that workers begin to expect it for basic or sub-standard services, to the point where people are afraid of offending for not tipping, stupid shit like tips being automatically built into bills...
Unless you are about to leave and won't be frequenting business again there shouldn't be much of a pressure to tip, unless you fear foid will press spurious harassment charges to get back at you for not tipping her.
If it's something like you plan to come back to a restaurant, I can see tipping as being like an insurance policy to avoid someone spitting in your food? It's like a protection racket.