The flu is a virus, which means that there is no cure outside of the your own body fighting it. Essentially, any fluids are said to be helpful in this process. Regular intake of fluids allows the body to expel waste faster, which in turn, allows the rest of the systems to be more productive. Like post #4 mentions, due to the nature of the flu and keeping things down, tea is something that is relatively gentle on the stomach and possibly more appetizing than plain water. The warmth of hot tea provides emotional relief in addition to physical healing, which also bolsters the immune system to fight the virus.
Antioxidants are common in many kinds of teas (Green Tea, they say, is especially healthy for the immune system), not to mention that drinking tea soothes the throat and makes you feel better in general, which is no small thing when you are sick. It is usually pretty easy to keep down if your stomach is doing flip flops, and herbal teas that are caffeine free are abundant and tasty.
The tea industry also claims that tea can help boost the strength of your immune system in general. They claim that theanine, an ingredient of tea, can help improve the performance of your immune system and help to fight off the flu.