If you are charging a battery the voltage across the battery stays relatively constant regardless of current.
Altering the duty cycle until you achieve the maximum current will put you at the point where the most power is being delivered to the load. This will be the point at which you are getting the maximum power out of the solar panel, wind turbine, or other similar device.
Assume a solar panel with an open circuit voltage of 24V and a 12V battery connected by a DC to DC buck converter where we have direct control of the duty cycle.
Obviously at 0% duty, no power is delivered. At 100% duty the panel is effectively connected straight across the battery, and we will assume that it delivers very close to the short circuit current.
As we reduce the duty cycle from 100%, the panel voltage rises, and the current reduces, but by a smaller amount. The buck converter is still producing the same voltage output, but the current into the battery rises (because the input power is higher). Reducing the duty cycle continues to increase the power to the battery until you reach a point where it doesn't. The controller needs to maintain the duty cycle at the point where maximum power is delivered.
If the level of illumination changes, then the power available will reduce and the duty cycle at which maximum power is delivered will also change.
The MPPT controller is always hunting for the optimum duty cycle to ensure that it extracts the most power from the source.