You can find this myth almost anywhere you read about repotting plants or growing vegetables in containers. Add a layer of gravel at the bottom of the container to “improve drainage.”
The problem is, this simply isn’t true. It’s not helpful at all, and it can also be harmful.
You’d be forgiven for imagining that by adding gravel to the bottom of your container, the water would pass through the soil and then drain down into the gravel, keeping your plant safe from getting “wet feet” where the roots are too wet.
But in reality, water does not drain well between different types of materials. What happens is that the water will actually drain more slowly as it reaches the layer of gravel. And worse, there’s also not as much soil available for the water to get away from the roots because of all the gravel in the bottom.
You end up creating exactly the type of situation that you wanted to avoid, and can make it so that your plant roots end up wet for a much longer period.
Still dubious? This video demonstrates how it works with gravel and some clear containers.
More References about Gravel and Containers
The Myth of Drainage Material in Container Plantings (PDF)
https://ucanr.edu/blogs/blogcore/postdetail.cfm?postnum=38681
The Hard Truth about Rocks at the Bottom of Planting Containers
