Gday Crispy
It's to measure water content.
If your moisture content is too high the honey may ferment after bottling.
If you only extract capped honey you will not have any issues and no need to use a refractometer.
However.....sometimes the honey is not capped, yet has a low moisture content and may be extracted. Some use a shake out method: nectar falls out, and what's left is honey. Using a refractometer takes out the guesswork when extracting uncapped honey.
It's also used for judging honey, where the honey moisture content must be below a certain percentage to enter...I recently entered a local comp and the cut off was 18%. I'm in the tropics and our content is usually higher than down south due to the humidity... So a refractometer can come in handy to ensure you meet the requirements. The honey with less content got higher points in this particular event.