2D VFX is part of the post-production and is tightly tied to other stages of the same phase: compositing, color correction, and final rendering; so much that the compositor and 2D visual effects artists of a project can be the same person.
2D visual effects are in fact flat simulations of events that originally occur in a three-dimensional environment. But most of the time there is no need to recreate them in 3D because they only need to move in the plane of a surface (the screen).
2D VFX can be of great help in fixing flawed shots. The production stage crew should do their job perfectly. But sometimes fixing things via 2D VFX in post-production is a lot more cost-effective than hours of re-working and re-rendering.
The sparks effect is designed for sudden bursts of sparks, such as when a bullet hits a metal surface.
Pixie dust is the kind of magical dust associated with pixies; fairy dust or any undefined means of working magic, in a fantasy world.
Dust is among elements that can greatly boost the atmosphere of a scene. It is a must-have 2D VFX for any post-production artist!
2D smoke is cheap, quick, and easy to apply into a 3D rendered scene. Creating the same effect in 3D is usually much more time-consuming and costly.
2D rain/snow effect is realistic enough for a wide range of applications without much trouble.
2D VFX is relatively more economical, simpler, and faster than its 3D counterpart. Moreover, certain functions like color correction, contrast adjustment, and compositing itself are 2D in nature. These effects can be more easily achieved in 2D.