Toolbox is designed as a place to represent the needs of tools connecting to a Fuchsia device. Capabilities needed by tools are routed to the toolbox realm and then tools can access this realm to use the capabilities.
The toolbox realm solves a platform evolution and stability challenge by creating an explicit, intentional location to make available capabilities intended for use by tools. Without a location the tools resort to connecting directly to the components that implement the capabilities the tools need. This creates a tight coupling between tool versions and platform versions, thereby making it difficult to reorganize the the platform component topology while maintaining compatibility. With the toolbox realm the platform component topology can change almost at will so long as capabilities are updated to preserve availability of capabilities in the toolbox realm.