Automatic brightness probably helps honestly. It could help confirm whether someone is in fact in an area that has high levels of lighting around them (e.g., in a store, at a beach on a sunny day, etc.)

Everything little piece of data that is gathered and used can help even if it isn't immediately apparent.

Now I could be wrong on this, but I feel like advertisers don't need to know something is true about a user, they just need to be confident something is true about a user and that's where data points like screen brightness can be of help to them.

Kind of a joke, but it could be useful for determining if they should serve light-mode or dark-mode ads. But I suppose they could just detect if dark/light mode are enabled.