The main "extra feature" beyond on/off is being able to trade battery life for brightness. Sometimes there's no substitute for a lot of light; if, for example, you are trying to do first aid, or looking for a dropped tool. On the other hand, more often having a lot of light is at best unnecessary, and having a battery life long enough to last the crisis is more important. How long that is is a judgement call, I suppose. Unless you are really diligent about keeping your batteries charged, you should maybe allow for the light being perhaps 75% flat when the crisis starts, so you need more duration than you might think. That said, you can often spend most of the dark hours sleeping.

Variable brightness is one way to manage that trade-off. I think it's good to have.

Flashing is another. I guess it's useful mainly if you are using the light as a beacon for others to find you: to be seen, rather than to see. Some lights can flash SOS automatically. I have trouble believing this is important. For me, flashing is a dispensable feature.

I guess the main thing now is that LED torches are so good you don't have to compromise. My Nitecore PD is a few years old now, and varies between about 2 lumen/50 hours, and 120 lumen/1 hour. Its piston-drive switch is very reliable, and I don't feel I've given anything up for the flexibility.