Inverter with low pass filter?
The part marked in red avoids propagating interferences due to the switching on the 12V DC line.

When one of the mosfets closes, a pulse of current is transmitted to the input via the parasitic capacitances present in the windings of the transformer and the circuit board.

This pulse is bypassed by the two capacitors, and stopped by the inductor.

Without that filter, the current pulses would be transmitted to the 12V voltage source via the external wiring, resulting in EMI radiation.