I know how it gets converted because PR is more stable and when there is neither red or far red light PFR naturally converts to PR. But what is the point of it? If PFR is the biologically active one why didn't evolution just make one pigment, PFR. At night neither is active so PR again has no point, in the morning PR gets converted to PFR. So why do we need a PR?
First off, "evolution" doesn't necessarily lead the most efficient system for anything - it works with what we already have. Once something has evolved, it might not go away, if there is no particular harm to having it around. You might take a look at the evolution tag, as other people have asked similar questions about the "usefulness" of biological mechanisms and molecules.
Aside from that, having both isoforms is important for the detecting the color of light. When Pr absorbs red light, it gets converted to Pfr, and when Pfr absorbs far red light, it gets converted to Pr. The ratios of Pr to Pfr allow plants to determine light levels and adjust their growth accordingly: