Not currently true in the USA, but in a lot of markets, there are places that charge a fee for using an ICE in say city centers...preserving the main battery can save a huge amount of money if turned on earlier so you still have sufficient battery power to traverse those areas entirely on battery power.
If your power use is significant when it turns on automatically at about 6%, you could run the risk of it shutting down certain functions if the REx can't keep up and the SOC continues to drop. Running fast, uphill, or with the HVAC cranked, all add to the power drain, and you can exceed the capacity of the REx to keep up. Having a bigger buffer can help.
Keep in mind, it was designed as a city car where your top speeds and distances aren't large, so exceeding the REx's capacity wouldn't be an issue when used as designed.
2014 i3 BEV, 2021 X5 45e
(The i3 will be sold soon, <17K-miles, interested?)