For what purposes are illustrations used ?

Illustrations are very commonly used in PowerPoints and novels to show a visual picture to an audience that uses visual learning to learn pieces of information.

Illustrations are also used for pie charts, bar graphs, etc.

An illustration is a decoration sorta it gives you a visual explaination of text,concept or process, designed for integration in published media, such as posters, flyers, magazines, books, teaching materials, animations, video games and films.

hope that helped :)