When out on a shoot, it's not always easy to view the camera's built in LCD monitor on the back. It's either not bright enough or too small for a group of people to see, plus, usuallly has very basic features to help you actually monitor your current situation for lighting etc.
Atomos' range of field monitors can also double as recorders in some models, like the Ninja V, which allows you to bypass recording limits on DSLR and DSLM cameras for indefinite recording length and sometimes, a bump up in colour.
SmallHD is now part of the Teradek family, as such, a few of the models of monitors now come with Teradek Bolt functionality for wireless transmission, so you don't have to remain static when monitoring or even operating the camera itself.