Why source cunneted to vdd in pmos?

In a PMOS transistor, the source is connected to the positive supply voltage (VDD) because it allows the transistor to turn on when the gate voltage is pulled low (below the source voltage). This configuration enables the PMOS to conduct current when the gate is at a lower potential, effectively allowing it to act as a switch in digital circuits. By connecting the source to VDD, the PMOS transistor can efficiently control the flow of current to the load connected to the drain.