"Purple Up" fills a niche in reefing somewhere between newbie/low calcium demand and intermediate/moderate calcium demand. It is popular, and it works, thats why every supplement manufacturer sells something like it. Specifically for Purple Up, it has balanced alk, ca, mag, iodine, etc to give bright pruple coralline. I use it on my FOWLR occasionally when I want that pretty purple color, but to use it on my SPS reef would be expensive and ineffective.
Just understand the spectrum or requirements for alk and ca and the most common way of addressing that need:
very low/low (purple up type additives/water changes) -> low (kalk) -> moderate (kalk/2-part) -> moderate/heavy LPS & clams (3-part/calcium reactor) -> heavy/very heavy SPS & clams (3-part + kalk, calcium reactor + kalk, kalk + 3-part + calcium reator and additional dosing)
So it depends where you are with your needs and how much you are willing to spend.