What is the overriding purpose of alternative fueled vehicle?

More than usually, the purpose of an alternative fuel is to increase the miles per gallon efficiency of an engine. a petrol or normal fuel car that makes 20 mile to the gallon can usually make around 40 miles to the gallon when it sports a diesel powered engine. Hybrids use an electric motor which is charged usually the same way the battery of a normal car is to basically assist the engine so it does not have t burn as much fuel to travel the same distance.
Purely electric cars just have an electric motor and therefore use no gasoline at all. In the future, there is promise for the use of hydrogen fuel cell vehicles which would be truly friendly to the environment which is another one of the motivations with which people buy alternative fuel vehicles.

Basically, to get away from fossil fuels and non-renewable energy sources.