I think the implication is you can use it to reflect additional light onto your photocell, improving the power output for a smaller cost than purchasing an additional cell.

Why aren't photo cells already manufactured to be like 90% mirrors then?

Some guesses:

- A lot more direction-dependent.

- More maintenance related to keeping the right direction, maintaining more moving parts, etc.

- Less convenient form factor, more susceptible to wind, more unsightly.

- Less space-efficient, which matters for some applications like house rooftop installations.

I'm not sure, but I suspect cooling requirements also enter into consideration.

Direction dependence also means your output drops even more on cloudy/overcast days.