Because of their simplicity solar-cell based systems have become lower-cost and easier to install and maintain for water heating as panel + electric water heater, vs a piped direct solar thermal heating system. Higher "efficiency" for the direct thermal system, but overall system costs are lower for panel + heat.

I still love seeing the interplay with different combinations of physical systems and clever things humans figure out. Including with solar panels + other system items.

Is this a good point to mention that simple solar heating circuits work better if you use diodes in series (for a fairly constant voltage drop across the solar cells) than if you just use a resistive element? :)