To achieve final or more intense color when heat is applied.

Most glasses are colored by adding metal oxides. 

For many of these glasses, the metal oxides work a lot like food coloring where the glass takes on the colors simply because the oxides are suspended throughout the glass.

Other colors, though, mature only after they have  been heated the first time.  These colors (typically yellows, oranges, reds an purples) may appear simply pale or even clear prior to the first full firing.  With these colors, the process of changing color - or maturing - is called striking.