Images that capture the entire data area of a storage media, sometimes also including unused blocks, boot blocks, file system periphernalia and other metadata areas can provide detailed peeks into the data that might otherwise be lost.
In some cases, such as old operating systems and early personal computer games in particular, copying the entire media may be the only way to ensure full preservation of the data, let alone the environment in which it lived. In other cases, it can later on provide interesting insights that nobody noticed
and which would have been lost had the entire media not been imaged.
Imaging the media also, as tjowens points out
, allows you to arrest degredation of the physical media itself. It also provides a nice alternative to storing a potentially large number of different physical media formats, including the equipment required to access those media artefacts. It's one thing to do something like access content stored on microfilm; it's quite another to be able to do something useful with that 8" floppy disk someone found in their father's attic; but from a preservation perspective, the data stored on the 8" floppy disk may very well have lasting value. If you can create a complete image of the floppy and store that on media that is more easily handled and migrated from if necessary, in many situations it might remove or at least ease the requirement of storing the physical artefact, and it certainly removes the need to handle
the physical artefact in daily use.