The two main reasons are aesthetics and service. First we add a tile border to add to the overall look of the pool. It should complement the other aesthetic features but does not need to “match”. It can be chosen so as to blend in with the pool’s interior finish as well. Secondly, it assists with the “bathtub” ring that can occur during the swim season with the bather waste (sun screens, body oils, etc.). By having a frost resistant tile at the water line, the area can be cleaned properly. Other type pools will put a “patterned” finish there so as to hide the bathtub ring. We offer a tile that will also do that but it can be cleaned as time goes on.