Tar and other deposits will condense onto any cold surface inside a stove, especially when burning wood. This can leave a black/brown deposit on the window of the stove that blocks the view of the fire.
A stove with an airwash system has air vents that direct a flow of air down across the glass which helps to stop tar being deposited on the glass.
Burning your stove relatively fast can also help to cut down on deposits as the firebox temperature stays high which means that much less tar is deposited. This can even burn off tar deposited earlier.