When 50 Watts isn't 50 WattsAs an example, consider a 50-Watt panel. The panel will typically be rated for full output power only at a particular voltage, say 17V, at which it will produce a little less than 3 amps.
When you connect this panel to your batteries with a conventional charger, the panel will operate at the battery voltage, say 12.6V. At this voltage, the panel won't generate significantly more current than at 17V, and the total output will be less than 40 Watts!
MPPT controller will let the solar panel run at its most efficient voltage, extract the most power possible, and charge the batteries at their voltage, with almost no loss. This way, a 50 Watts power can be received from the 50 Watt panel, a 25% gain over the conventional controller in the example above!
MPPT controller extracts the most power possible from the panel in nearly any condition: hot or cold temperatures, different light levels, partially shaded panels; it even takes wiring losses into account. The actual power gain will vary with conditions, and may be anywhere from 0-40%, with 10-30% being typical. Highest power gains come with discharged batteries, cold temperatures, and lots of light.