As someone who has worked in a restaurant or two, I can honestly say that the number of people that do not tip the server is astonishing. Lately, more restaurants have been paying servers a livable wage so that tipping is not necessary. However, this is not the case for most restaurants. In Virginia, for instance, servers are paid $2.13 an hour plus tips. In some restaurants, they are even required to "tip out" hosts, bartenders, bussers, etc. This essentially means that they have to give a portion of their earnings to each of the aforementioned people. At some restaurants, the servers can choose how much they give, but at others they are required to give a portion of their sales (this can take a server from a $60 night to a $40 night). These restaurants may even provide credit for the hosts and bussers. For example, if a host makes $5.35 an hour and works for five hours, then he makes $26.75 for the night. If his tip-out does not make his income $36.25 (minimum wage for five hours) or more, then the restaurant makes up the difference. However, this is not the case for servers.
Not only do servers get drinks, food, and condiments for their customers, but they do side work, which includes a lot of things that people don't usually consider. They roll silverware, marry sauces, fill salt and pepper, they clean the dining room and a myriad of other things. This is a job well worth minimum wage.
Many people ask why it's the guests' responsibility to help servers pay their bills. It's simple: You pay for services. At the end of the meal you get a bill, which is what pays for the food. However, the service provider is someone totally different. It's just like paying a mechanic for the parts plus labor. In a way, the server is in a completely different business than the restaurant.
Bearing that in mind, look at it this way: If you sit in a restaurant for an hour, you technically owe the server $7.25 for an hour of their time. However, most servers ask for 15-20 percent of your bill. If your bill is $20, then the tip will only be $3-4.
Considering all of the trouble that servers go through, it is only right that we tip them.