I am also confused by this. I've heard from many different places, and experienced myself I think, that fasting seems to make all your senses sharper and generally makes it easier to focus. And I've heard that this is because when you're hungry it's important in the EEA to get food, and focusing helps towards that.

But then why doesn't evolution just make your senses sharp all the time? The best hypothesis I can come up with is that having such sharp senses is bad or unsustainable over the long term.

In which case you probably shouldn't fast just for the heightened focus. Or maybe not much more than people in the EEA did.

(Another hypothesis is that people were hungry enough in the EEA to basically be in this state of heightened focus all the time. But then why turn it off as soon as you get food in your stomach?)