Solar panels are made up of individual cells. Each cell puts out x volts and
enough cells are installed to typically output 15-19 v AT OPEN CIRCUIT. Note
the shouting. Every panel has an amp vs voltage curve. At open circuit
conditions the amperage is zero. The maximum wattage (power) point on the
curve (volts x amps) is about 15-16 v. If one fewer cell were installed this
curve would be x volts lower and too low to effectively charge your battery.

If you simply connect the panel to the battery, the battery will load the
panel down to about 13 v which is below the maximum power point. But it is
still pretty high. Maximum power regulators load the panel to its maximum
power point and then convert the voltage down to about 13 volts to charge
the batteries. This typically can increase power by 10-15%.

David