Water changes are for removal of nitrate and other salts. This keeps the fish healthy and water conditions clear. The specific frequency and amount of water changes depends on the water source, species, plants, and stocking level. A heavily planted, CO2 injected, lightly stocked tank using RO water to top off may only need a water change every few years. A overstocked goldfish tank might need 50% every other day.
When I was keeping my 125gallon tank in a drought prone area, I routinely used the water for my flowers and garden. Stuff I was going to water anyways and never worried about 50% water changes as it didn't impact my overall water usage.