You want to be able to enable features for certain customers. For example, you might want to roll out a feature to only the US or only English speaking users because you haven't internationalized it yet. Or you might want to enable it for select customers. Or all sorts of other examples.

You may also want the ability to gradually roll out a new feature over time. This lets you test in small scale and also lets you ramp up load on new backend services.

You can also schedule features to be released in advance. This helps you align releases with things like marketing or customer service training.

I only hit on a few points, but you can see it's a lot more than boolean flags.