Maybe I shouldn't call it dead zone, bad wording on my part. But essentially over time when growing in pots after each water no matter what you do it will start to compact down and at the bottom it will always retain moisture, ie. a perfect breeding ground for gnats and anything else. I've found it also helps promote proper drainage. It's only a very small layer. Like 1" of chunky perlite. I've noticed a much bigger root mass on the girls I've done it with and also gives me a nice even dry/water.
99% of the time by time harvest comes it's all been pushed into the medium anyways so it helps aerate that bottom section for me.
My grandma taught me this many many years ago and it just stuck and now I use it with good results so I keep doing it.
I'm not trying to convince anyone to do it or saying it's right or their way is wrong. Just curiosity, cause how will you ever learn without asking questions