Hex bar is the one that wraps around the lower body and the lifter “steps into” to deadlift compared to a traditional barbell that is straight where the lifter picks it up from in front of them. Typically doing hex bar deadlifts works your leg muscles harder than a traditional deadlift because the movement is very similar to a squat, whereas a traditional deadlift typically very strongly works just your back/glutes/hammies.
My guess is they are doing hex bar deadlifts for the following reasons:
1. Prevent lower back injuries as there is a lot less stress on your back with hex bar deadlifts and more focus on legs
2. Strengthen the area around the knee to prevent knee injuries since more leg muscles are being worked with hex bar deadlifts.
I’m not a strength and conditioning professional, just a gym bro who has done a lot of research so I could be off.