The spiritual and mysterious quality of light is an important element of the religious symbolism of Gothic cathedrals. The people used the light not only for decoration of the churches or cathedrals. There are several purposes why they used the role of light.
First, the light was used for practical purpose. The use of windows and natural light allowed their cathedrals and churches to shine from the inside out. So, this kind of light was economical. As the churches grew in size and scope, it would have been harder to light up and get enough lights from the candle light. It was also dangerous as well because the buildings were made of wood and it might catch fire. They might have thought about cost of using large amount of candle for enormous buildings and also, risk of catching fire.
Second, people were always looking for artistic and impressive object for churches or cathedrals. The architects used the beautiful art form of the stain glass to emphasis and work with the light. The stain glass windows and light gave churches bright and heavenly atmosphere.
Finally, the people in this era were still illiterate. The art with light were very effective way to impress people who don’t have ability to read. These stained glass windows worked to tell the stories of the Bible, so it was convenient for missionary as well as people who cannot read.
To conclude this, gothic art and architecture focused on the role of light because light has always been important to the Church. It gives churches practical benefit and artistic and symbolic benefit.