When you tip for good service when provided, the server keeps that tip. But that does not take into account all the other people who made that good service experience possible. If your meal was tasty and the way you wanted it, did you tip the cook in the kitchen? After all, they prepared the meal. All the server did was take your order and deliver it.
Were the cutlery clean and plates spotless? How does the dishwasher benefit. Do you tip the dishwasher? And if the atmosphere at the restaurant helped to make the meal a pleasant one, do you search out the groundskeeper and tip them?
Since the US system was directly brought up, the US allows tipped employees to be paid as low as us$2.13 an hour:
http://www.dol.gov/whd/state/tipped.htm
But unlike the US, servers in Jamaica do not have a special lower wage, they are generally paid the same as everyone else based on their experience and benefit to the business. The service charge goes a long way in providing a practical way for more than a single individual to benefit from your dining experience.