Wall Worm is a script and tool developer focused on 3ds Max and various Max-related pipelines. Historically, Wall Worm was devoted soley to optimizing the Source Engine asset pipeline. The Wall Worm toolset itself is a Source-only toolset. However, many new tools are generic in nature (for any game engine, VFX, VIZ, etc), such as the CorVex plugin and most other new tools.
See the official Autodesk blog feature about Wall Worm developer Shawn Olson, Wall Worm and Black Mesa: From Black Sheep to Black Mesa and Beyond.
Wall Worm tools allow you to create assets for Source in a way that was not possible before. You can build models, textures and levels inside one environment (3ds Max) and compile them straight into Source. Wall Worm tools have been used in the pipelines of many games including Dear Esther, Black Mesa and by designers of CS:GO Operation maps and many more.
Wall Worm works in collaboration with other 3ds Max/Source developers to help enhance all the tools available to designers and mod teams.
When reopening a scene the models can appear as cubes if the game settings do not include paths to the source MDL files.
This document explains why models can appear as cubes instead of props when importing VMF or MDL nodes.