Why do solar heaters need glass to cover it?

Question

actually exact question is: What is the purpose of covering the solar heater with glass...

but its the same..

Answer 1

The glass creates the "greenhouse" effect by allowing the infrared (heat) rays into the solar collector, but blocking the heat escape.  This concentrates the heat on the collector and makes it more efficient.  

The glass also blocks the wind from stripping the heat away from the collector.

Answer 2

The glass allow the heat to come in but it does not allow the heat to get out.This is the principle of green house

plastic sheets and glass are useds for this purpose.

Carbon dioxide is also a good green house gass!