If you want to bottom-water a pot using a wicking systems or capillary mat, that drain hole is a critical resource. The capillary mat needs to touch the soil THROUGH the hole.
Or, you can loosely fill the hole with something that wicks (cotton fabric, acrylic felt, acrylic yarn or maybe braided nylon parachute cord. That might leave some room for some soil to escape, but it will provide a "capillary bridge" or wick between the watering pad and the soil in your pot.
Of course no wick is needed if you bottom water by immersing the pot in a few inches of water.
The bottom hole is also valuable if you accidentally over-water. For example, heavy rain might have flooded the pot. If the plant can't drink drink all that water fast enough to keep roots from rotting, set the pot down on a towel that dangles down even lower than the bottom of the pot. Now gravity is added to capillary force, and excess water is pulled down and out of the pot much faster than it would have drianed by graivty alone. This even remove4s perched water.
I wish I remembered who taught me the phrase "wick-in, wick-out"!
