What is the reason for using the transistor (2N3904) is this circuit? What good is this transistor doing in this circuit.What happens if we remove the transistor from this circuit? And what will happen if we use a PNP transistor instead of a NPN transistor.

so my explaination about the original circuit which I implemented because I don't have 50 OHm resistor.When there is light in the room, the circuit inside LDR is complete and hence all the current flows through 1K and LDR branch, and very low current is at base so the amplified current by 2N3904 is insufficient to lightup the LED. When there is dark in the room, the circuit of LDR is open and hence the base current of 2N3904 is maximum (2.3mA) which after amplification is 232.2mA which is sufficient for the LED to work. The LED has voltage drop of 1.8V.