Iron sharpens iron.
The old adage is especially true for MMA. If a fighter wants to get a feel for the high-stress, high-intensity dynamic they will experience when they step into the cage for real, they have to get used to fighting in the gym.
Sometimes more restrained sparring is used as a device to refine techniques, and to an outsider can often look more like a dance than a combative contest. To quote Conor McGregor's coach John Kavanagh, the goal of sparring should be "upgrading the software without damaging the hardware."
Not all gyms adhere to this philosophy however. Several MMA teams have become infamous for their frequent hard sparring, leading to the fighters accumulating concussive damage before they've even stepped into the cage.
In order to become accustomed to fighting at the pace required in an MMA fight, fighters will regularly attempt to replicate the intensity seen in the cage. Add a camera to the mix and you have the perfect recipe for some of the craziest fights that have ever been filmed behind closed doors.