The day tank is the least glamorous part of a chemical feed system and the one most likely to cause a problem. It holds the chlorine or acid your feed pump draws from, and it has to do that safely, without degrading, without spilling and without letting anyone mistake one chemical for another.
High density polyethylene resists sodium hypochlorite and muriatic acid over the long term, where metal corrodes and lesser plastics become brittle. Containment matters because the failure mode you are guarding against is not the tank splitting open, it is a fitting weeping onto a concrete pad for a week before anyone notices.