Encryption provides the ability to communicate without any intercepter being able to read the content. In many cases this is fine, but in some cases using encryption software is enough to incriminate you (e.g. spies).

Hence steganography means that not only can your messages not be decoded, they can't be detected.

Specifically, this extension's innovation is allowing steganography on walled gardens such as Facebook which recompress images after uploading them.