Why does the PR form of phytochrome exist?
First off, &quot;evolution&quot; 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 &quot;usefulness&quot; 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:


Sunlight has a R:FR ratio of 1.2

Light under a canopy of leaves has a R:FR ratio of 0.13

Light under 5 mm of soil has a R:FR ratio of 0.88

A higher proportion of FR light allows plants to detect when they are
shaded. Plants adapted for growth in full sun will display greater
stem elongation when they are transferred to shade.  They also develop
smaller leaves and less branching.  This change is due to greater
proportion of Pr to Pfr.

Seeds of certain plants require red light for germination; FR light inhibits germination.  Many small seeds with low amounts of storage reserves (such as lettuce) show such a red light requirement.
If these seeds they are buried below the level of light penetration in the soil, they do not germinate.
If they are shaded by a leaf canopy, causing a high proportion of FR, germination is inhibited, [as] Pfr is required for germination.



Source: Missouri Botanical Garden