Photovoltaic modules (solar electric panels) generate a current with a much higher voltage than the battery voltage. If a module is left permanently connected to a battery, the voltage of the system may rise to a level above 15 V. Lead-acid batteries do not tolerate such a high voltage for long periods. A charge controller helps keeping the battery in a fully charged state without allowing an overcharge that might damage the battery.
An MPPT (Maximum Power Point Tracking) controller can sense the maximum power voltage Vpm. For the Aurinco Bluewater and Compact panels this is 19V, much higher than the battery voltage. The MPPT circuitry allows the solar panel to operate at this most efficient voltage while transforming the higher power into more current (amps) at the battery voltage level.
Some charge controllers may have additional functions, like protecting the battery against deep discharge. A discharged lead-acid battery will rapidly acquire a sulphate deposit on the surface of its inside electrodes, which will diminish the capacity of the battery and its life.