Inducer motor for starters. This blows hot gasses outside. Circuit boards need power, the switches and relays on the boards. There’s programming and timings that are on the boards. Most models now also have wifi modules too.
I’d rather be able to shower with no power tbh…specifically opted for at.ospheric for that reason. Much cheaper to buy upfront and works in the event of big storms etc… tankless can suck my dirty nuts but I see the appeal, kinda…