I do not expect anyone to agree but I have no self control and will say it anyway:
Full disclosure: I do not have a solar system or have any experience using a solar system of any kind.
A 100 watt solar panel has approximately 5 amps of production and a deadhead voltage of around 19 volts on a very sunny day with some allowance for dust, less than perfect sun angle, and trying to keep the numbers decimal free.
1) A 100 watt solar panel hooked up to a 6 volt battery with no solar controller and good wiring will send 5 amps to the battery or 30 watts with a good sun exposure (keeping my example simple by ignoring charge state voltage versus discharge state).
2) A 100 watt solar panel hooked up to a 12 volt battery with no solar controller and good wiring will send 5 amps to the battery or 60 watts (keeping my example simple by ignoring charge state voltage versus discharge state).
3) A 100 watt solar panel hooked up to two 12 volt batteries in series with not solar controller and good wiring will produce zero amps since the panels will be "deadheaded".
On a modestly cloudy day, a 100 watt solar panel is capable of producing 4 amps and has a deadhead voltage of 17 volts.
4) On a modestly cloudy day, 100 watt solar panel will send 4 amps to a six volt battery with no controller and good wiring or 24 watts.
5) On a modestly cloudy day, a 100 watt solar panel will send 4 amps to 12 volt battery with no controller and good wiring or 48 watts.
My Ford F-150 alternator hooked up to a 6 volt battery with good wiring and no controller or fuse will produce 130 amps and boil the 6 volt battery or burnout the alternator or both.
The alternator is a fixed voltage device with variable amps.
The solar panel is a fixed amp device.
An MPPT controller goes after what are called "lost watts". In example 1), the lost watts are 95 (19x5) minus 30 (6x5) or 65 watts. In example 2), the lost watts are 95 (19x5) minus 60 (12x5) or 35 watts. In example 4), the lost watts are 68 (17x4) minus 24 (6x4) or 44 watts. In example 5), the lost watts are 68 (17x4) minus 48 (12x4) or 20 watts.
MPPT controllers do not create watts for free. They consume approximately 10% of the input watts to operate. Given the difference between battery charge and discharge voltage and the 10% watts lost across the MPPT controller, MPPT controllers are especially effective above 17 volt deadhead conditions, i.e. getting lots of sun.
MPPT controllers periodically disturb the current/voltage coming from the solar panel to calculate how many watts are available at the current solar level and then do a condition specific DC
C conversion to convert the unused watts to usable amps/volts for charging.
A very good MPPT controller could charge the two 12 volt batteries in series but at around 2-3 amps from a 100 watt panel on a sunny day.
Expensive but very smart devices.