One reference: http://www.solar-facts.com/controllers/mppt-controllers.php

"How Does MPPT Help"

An MPPT controller, in addition to performing the function of a basic controller, also includes a DC voltage converter, converting the voltage of the panels to that required by the batteries, with practically no loss of power. In other words, they attempt to keep the panel voltage at their Maximum Power Point, while supplying the varying voltage requirements of the battery.
Furthermore, a 24 volt system with an MPPT charge controller may have the panels wired in series to produce 48 volts, maintaining the ability to provide some charging current in dull conditions, when a standard system would not provide any charge.

Manufacturers claim up to 40% power increase from your panels using MPPT, which is most likely to be achieved when battery levels are low and/or light levels are low."

I added the bold effect. In order to charge batteries you need to start with a higher voltage than the nominal battery voltage. Volts = electrical "pressure", and electrons will always flow from higher to lower. That page refers to a 24v system, but the way I see it -- if you have a 12v system and run 34+ volt (no load) PV panels (2 x 17v in series) there is a greater differential voltage than if you run 17v (no load) PV panels. This gives the MPPT controller more "headroom" to work with when in lower light, in converting power to amps.