Serious video producers don't just rely on onboard camera microphones.
When filming a wedding on location a combination of mics comes into play. I may need to discreetly mic up a groom for during the ceremony so the vows can be clearly heard. At the same time, I could be using a condenser shotgun mic to capture ambient sound. And even for the speeches, I could be using 2 radio mics for each end of the top table for balanced audio capture as well as a shotgun mic aimed at the top table.
An audio field recorder allows me to record each mic into separate track files but at the same time sending the audio directly to the camera already mixed down into a stereo channel. This allows me to manipulate the different mic sources during editing if necessary. And to have a backup recording of the most important parts of the day, where audio needs to be clearly heard without the distortion caused by unwanted noises.
The Zoom H6 is the recorder that I choose for wedding shoots as it is packed with so much technology that just makes my life so much easier when filming a wedding. It is light, runs on batteries which can last up to 20 hours. And can record up to 96kHz / 24 bit audio for amazing sound capture.