Most of brushless controlers do generate a lot of electromagnetic noise (EMI/RFI). 
During FPV, this is a serious issue that may lead to a video or control link failure then eventually a model crash/lost.

Generally speaking, when motor is running you will get a distorted picture in the video goggles,
or a reduced radio range (see picture below).

These noise suppressors will get rid of these interferences,
and provide a strong and secure video / radio link.
They connect between the battery and the device to filter (ie: video camera).