Construct 2 is great for a no coding game environment, but there are implementations and game logic that is best handled in the code level. Basically Construct 2 has been fantastic in handling predefined game design, behavior and system development workflows. Below are the few reasons where custom plugin are needed.
1. AI System. If you really need a high performing AI, it is best to create it as an independent plugin.
2. Puzzle Game Computations. Certain puzzle require matrix matching or elimination. This is still best done in matrix array through JavaScript and let Construct 2 help you handle the rendering portion.
3. Graphs and Analytic. Currently Construct 2 don't have this system since it is not common in 2D games, but if your game requires a custom HUD (heads on display) with graph data on it definitely you need to create a custom plugin.
4. 3D interface. Construct 2 is mainly designed as a 2D game, but some designers wanted a 3D environment tiered to a 2D components. To do this you need to create a custom plugin and use extend third party libraries such as Three.js framework.
Scirra has a good working and supportive community in the forums for making Construct 2 games, to know more of Contruct 2's Plugin and Javascript Extensions kindly checkout this link. Check the original blog post on this link.