Browny I try to explain a solar system like a rain water system

the solar panels are the roof, the bigger the roof the more rain you will collect (when it is raining)

the batteries are the water tank, the bigger the water tank the longer you can go without rain

the applinace/lights are the taps, the more you turn on the taps the bigger your tank must be

the regulator is the gutters and down pipes, they send the water to the tank, if they are under sized you could waste water when they overflow in a good rain shower,
a MPPT reg is like putting on extra large gutters and down pipes, it will cost more but you will never waste water

if any part of the system is too small you will require some back up