Retro
C2 can already do a lot without any plugin.
But there are still some feature or ways of doing things in javascript that you can't access with C2 defaults plugins.
For instance:
- associative arrays: a way to gather data (character stats, highscore, etc) as a pair of key,values you can access the value by calling the key. In some case it can be really usefull. That's why Mipey created a Association List
- canvas: with the default build of C2 you can't really manipulate pixels one by one (unless creating some 1x1 sprite with 16777216 animation frame (one by possible color) and spawning them where you want :D so yeah for that, plugin are usefull
- bitwise operation: you can't use javascript bitwise operators (AND, OR, NOT, Left and Right Shift). Ok bitwise operations aren't often usefull but I did make a plugin for that :D so well... Bit
Look at all the plugins there already are. By reading their description you'll see that they cover some intersting side c2 doesn't... yet (for some)