I'm desperate, trying to solve an N-Channel MOSFET problem

I managed to POC something with a P-Channel FET, 2N3904 and NDP6020P.
Can you please help me understand this practice ? why do I need the 2N3904 for ? Can't I just trigger the MOSFET directly from the arduino pin ? Why do I need 2 switches in that manner ? @6v6gt @TomGeorge