Some accessories (such as docks and certain speaker systems) are designed around the iPod dimensions, which can vary from model to model. Instead of having to release multiple versions of an accessory for each iPod size, companies can release one that works with different size dock adapters.
Note: The iPod nano Dock Adapter that ships with the iPod nano (Second Generation) will not fit earlier iPod nanos. The iPod nano Dock Adapter will also not work with docks designed for earlier iPod models, including iPod with color display and iPod mini.
For information on identify dock adapters and what iPod models they work with as well as information on ordering additional dock adapters see this document.