Question: Do gravel or broken flower pots in the bottom of a flower pot improve the drainage?

Answer: No – no more than gravel in a stopped-up bathtub would improve its drainage. No commercial growers use that technique anymore. What it might do is to keep roots from exiting the drain holes and growing into soil beneath the pot quite so freely. You can accomplish the same thing, however, merely by moving outdoor containers once in a while, so that roots can’t grow out of the holes and into the soil, also so that the drain hole doesn’t become plugged with compacted soil. With your houseplants, gravel in the bottoms of the pots does nothing more than take up valuable space that soil should be filling.