Sheet PVC is a very useful material for fabrication of a variety of nerfy things. It's easy to carve, amenable to solvent welding, easy to thermoform, lightweight, and strong. This makes it ideal for making switch mounting brackets, internal structural pieces, bridging gaps in shells, shims and fit adjustment, rapid prototyping . . . the list goes on. While blaster shell scraps can also be used for these purposes, sheet PVC is often more convenient due to being thicker, more readily available with larger flat areas, and of course much easier to thermoform than ABS.
I've taken to preparing sheet PVC in bulk. It's much more convenient than thermoforming each little piece of pipe as-needed, as flat sheets are usually the shape that I want to start from when making each PVC part.