Question regarding a Furrion Tankless H2O heater

[Texas Sooner]

I am in the process of descaling the tankless water heater in my Imagine 2970RL. I'm following the Lippert video instructions showing how to properly descale it When I disconnected the plastic coupling of the hot water line from the brass hot water outlet on the heater, I noticed an end piece protruding from the brass outlet that appeared to be some type of rubber-like material that was torn and hanging down, so I pulled it off. I've attached a few photos of it.

I attached the hoses I'm using for descaling and went on with the process, but started thinking about why this material was only on the hot water outlet and not the cold water inlet. All I could figure out is that it might be some type of material to prevent the plastic coupling from having direct contact with the brass outlet, which would obviously get hot once the tankless heater kicks on. I'm not sure why they would have a plastic coupling attaching directly to a brass hot-water outlet anyway. Time is not of the essence, as our next trip is a month away, so I'm not going to reconnect the hot water line until I'm sure how to proceed, since I don't want the plastic coupling to heat-stress and crack. So, any ideas, thoughts, or suggestions regarding this would be much appreciated.