What is the reason behind the mosfet setup in circuits similar to the one shown in the image?
The core of that schematic is this:







simulate this circuit &ndash; Schematic created using CircuitLab

That's the schematic of a step-down (buck) converter. Your schematic has two, one for 3.3V, and one for 5V.

The drive MOSFET is turned on for a short time and runs a current from +BAT through the coil into the +3.3V output. Then, it's turned off and the body diode of the commutation MOSFET takes over the current through the coil 
L, which cannot be turned off in an instant (it's a coil!). The commutation MOSFET is turned on to limit the losses introduced by its own body diode. Then, the commutation MOSFET is turned off again and the cycle begins another time. The control checks the voltage on the output and turns on/off the mosfets so it's regulated to 3.3V.