There are a few reasons to refill an existing pool, too. For one, you should drain and clean your pool every two to seven years as regular maintenance. The better you are at keeping your pool cleaned and treating it with the right chemicals, the longer you can wait for the next draining and refilling.
Another reason is that you might drain the pool to make necessary repairs. Pools can get damaged over time, and they build up chemical deposits and collect debris. You might choose to refinish the bottom of your pool to give it a newer, fresher appearance.