Yes, that's the million dollar question here. Why do we have a power restore tool associated with our healing trait line? Why does said "power restore tool" have a feature that increases healing if it's only supposed to assist with power restoration?
The obvious answer is that it's supposed to be a healing tool, but it wound up being not very good at that. Trying to push Song-Brother as if power restoration is the primary feature is selling the skill and ultimately the class short, because it was meant to be much more than that.
Furtim that's not a "million dollar question". Frankly it's common sense.
Power is extremely important to healers - because if we run out of it the group wipes. So it makes perfect sense why Song Brother would assist healers with power - especially in this context regarding the capstone trait that allows us to share in the effect.