What is the purpose of convection currents?
Convection is one of a mechanism through which a system attains thermal equilibrium.
Thermal Equilibrium: A system is said to be in thermal equilibrium if all parts of the system are at the same temperature. One can see temperature as the concentration of thermal energy. If the thermal energy concentration is not uniform, then energy flows from regions where it is more concentrated (higher temperature region) to regions where it is less concentrated (low temperature region) unit its concentration is uniform throughout the system. So attaining thermal equilibrium requires flow of thermal energy from one point in space to another.
Flow of thermal energy is called heat. There are three modes of heat transfer.
 Conduction: Thermal energy is transferred by the vibrations of the medium particle. Though the medium particles move (vibrate) there is no net flow of medium particles in this mode. This is the dominant mode of heat transfer in solids.
 Convection: If the medium happen to be a fluid (things that can flow), then the medium particles themselves can carry thermal energy and deliver it across. There is a bulk flow of medium particles in this mode.
 Radiation: If there is no medium between two points, thermal equilibrium is achieved through thermal energy transfer in the form of electromagnetic waves. This is called Thermal Radiation.