I don't understand why Brave Rewards needs to be enabled, but it seems very normal for the company operating a service to require some kind of account or membership. If you don't trust Brave this is probably not the service for you.
In a nutshell, enabling Rewards is their incentive for giving the product away for free, otherwise they charge via Premium.
They just require you to allow their ads during your call. It's privacy preserving ad tech in exchange for 1:1 video calls.
That's way better than sites requiring you to allow 3rd party ads to use their site.
EDIT: I just saw they let you set your ads to "0 per hour" and still let you use this service. I wonder if that's an oversight.