When you need a bit more light and a bit more control
Design is really up to you.
Since each LED has it's own resistor, the resistor is chosen based on the source voltage, the voltage of the LED and the brightness you desire.
Most individual 5mm or 3mm LEDs can work with anywhere from 0 (off) to 20mA (Full Brightness), so select the brightness you want.
For most red, orange or yellow LEDs, they turn on at about 2 volts. Most others turn on at about 3 volts.
If, for example, you are powering this board with 3 AA alkaline batteries, you would have about 4.5 volts.
Resistor = (Supply voltage - LED voltage) / Brightness. For a Red LED at half power Resistor = (4.5 - 2)/ 10mA = 250 Ohms. Since that's not a popular value, maybe try 220 ohms. It's close but will be a bit brighter.
From there it's up to you how many LEDs, what type and how to lay them out.