To give them the best possibility of growth and good development of the body, fins and organs, they need exceptionally high quality water. It's also thought that dominant fish may produce growth inhibiting hormones that prevent other fish in the tank from growing, and this can also be removed by water changes.
Once they're fully grown, their development is complete.
Think of it like giving a baby human nutritious milk and food, protecting it from germs, etc., but adults needing less protection and being able to tolerate lower quality foods and higher numbers of germs.