It's not.
A breeding tank should have a substrate, plants or caves (depending on species of fish being bred), filter, heater if required, coverglass, and it should be full.
If you are breeding fish that scatter eggs among plants or in the substrate, then you remove the adults after breeding and reduce the water level. The reason you lower the water level is so the fry and fry food are closer together. If you have 20 baby fish in a 2 foot tank that is full, the babies have to swim long distances to look for food. By reducing the water depth to about 4-6 inches, the fry and food are much closer together and the fry are less likely to starve.
As the fry grow and become stronger swimmers (usually after a couple of weeks), you can increase the water level.
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If you are breeding cichlids or other fish that show brood care, then leave the tank full because the fry are usually bigger than fry from fish that scatter their eggs among plants, and the parents keep the fry together so they can be target fed. This entails using an eye dropper to direct fry food to the fry.