Quoted from bobukcat:
The inserts for the villains light steadily brighter as you progress towards completing them (dim, brighter, fully lit). If you put LEDs in without LEDOCD they are either on or off. Like I said it's not a big deal to most people but it would bother me a bit.
Also, the 2X and 3X white inserts have different brightnesses that you can't tell the difference if you don't have LEDOCD.