his multiple pipes collect more heated air because of the multibaffled system- the air moves over more surface are than just a plan glass covered box.
even a glass box needs some diverters to slow the air enough to pick up heat- otherwise its just cold air in and "cool air out"
especially if it has a fan/thermostat.
the link above will show you better than i can tell you.If you live where you get alot of sun these will heat your whole house. I will only get about 4 hours of sun a day, but that is 4 hours i won't have to use electric heat.