Since most packages have similar webpack configs, and since every project package ends up being a piece of a larger year package, it makes sense to have a common webpack configuration to prevent configuration drift. This way we can be sure that integrating a project with a year package will go smoothly.
Since it isn't realistic to expect every webpack configuration to be identical, due to potential differences in entry points and outputs, webpack-common isn't a simple file. It's a set of functions and blocks from which configurations can be composed. This allows more control than a single rigid export, but less control (and therefore less potential for drift) than webpack with no abstraction.
Webpack Common is built on top of Webpack Blocks. Webpack Blocks takes care of the details involved in merging multiple webpack configurations.