Pictorial allegories, which were common paintings during the Middle Ages, were important because most people during that period (from about the fall of the Roman Empire in 476 c.e. to about 1500 c. e.) could neither read nor write, and about the only way to transmit any kind of knowledge to the widest possible audience was through paintings.
Because most art during the Middle Ages was religious in nature, often depicting biblical scenes or entire biblical narrative stories, many medieval artists used their paintings to transmit allegorical stories to the masses. For example, a common allegorical painting took as its subject the Seven Deadly Sins (lust, gluttony, sloth, etc.), and the painting would show common people in common settings engaging in these sins. This was the only practical way to transmit biblical injunctions on behavior to the average medieval peasant who couldn't read but could understand a visual representation of an allegorical scene.
Until literature, including the Bible, was translated into vernacular languages (e. g., German, French, Italian, Spanish rather than Latin), and the printing press made books more readily available sometime after 1500, paintings constituted a primary method of disseminating cultural information, and in the Middle Ages, religious information was very important.
Thanks a lot! :)