The term 'Module Carrier' may sound strange -- but once you see a picture or actually use one, it makes sense. Here's a picture...
There are many pre-built circuit Modules worthy of Breadboard real-estate. But they typically don't mount well to Breadboards.
Carriers solve this providing a platform to hold the Module -- and pins to plug into the Breadboard.
Carriers make life easier since they can be moved to a convenient location -- or removed later if project goals change.