Why? You're shortening the pot and you're raising the perched water table.
If you're interested, check out
@syenite's post on the "
perched water table," with a great video demonstration as well! Start at 6:32 if you're pressed for time after reading the post. It shows how rocks raise the perched water table making for a shallower pot.
Here's a pretty in-depth page on it as well:
There’s an old gardening myth that it’s best to put a layer of gravel or rocks at the bottom of a plant pot to improve drainage, but how true is it? Can the practice actually harm plants more than …
deepgreenpermaculture.com