In some cases, you might want to use it with PIC24F and also dsPIC devices that needs 3.3V. Thus, it is better to have both 3.3V and 5V.
Figure 1: 5V using 7805
Figure 2 and figure 3 shows a simple circuit that will provide 5V and 3.3V. Here, a voltage divider is used to convert the 5V to 3.3V. The switch, SW1 can be replaced with a 3-pin connector. Thus, this will enable you to have, 5V, 3V and GND on the 3 pins.
Figure 2 shows direct output 5V from the regulator while Figure 3 shows the output from the regulator is passed through a voltage divider to get 3.3V.
Figure 2: Direct output from regulator
Figure 3: Output from a regulator through a voltage divider
Having 5V and 3.3V, power supply could be very handy. IC's such as PIC24F would require 3.3V while Max232 requires 5V. Therefore, in this case, you would require both 5V and 3.3V