What's a workbench?
It can be equipped with source code repositories which can be iterated, inspected and generally worked with.
A workbench can also be equipped with further tools in addition to what the core provides. Tools are scripts that can be executed as workbench subcommands and they have access to the extensive scripting library and facilities provided by the workbench (configuration, project info, repository info, VCS abstraction, Build system abstraction, notification, and more).
Tools can have several origins. The core workbench provides tools for the most basic tasks, like updating repositories, or accessing the configuration. Projects can provide tools, users can write tools, and tools can be downloaded from an online Tool-Shed.
A workbench is able to combine multiple projects, and projects can continuously provide all sorts of workbench customizations.
A workbench instance provides a coherent place to work with one or more projects while tedious repetitive tasks are automated.
There is an exploratory implementation that can give you a feel how it could work, and get your creativity started about the multitude of possibilities a full workbench implementation would enable.