Before projectors entered the education market, if a teacher wanted to show a presentation, students either had to take turns looking at a small monitor or the educators had to deal with frustrating conversion cables to try and run the presentation on a TV screen. By the same token, notes written on a board are very difficult for the people in the back row to read in a 500-seat lecture hall. A projector enables educators to easily display information so that all students can read it clearly.
Projectors are useful because they enable a teacher to show students a variety of media through one easy-to-manage piece of technology. There is no need to switch between an overhead for displaying notes, a slide projector for images and a film projector for video. Instead, all of those types of media and more can be displayed through a laptop hooked up to a single digital media projector.
Projectors allow educators to take lessons to a level that is simply not possible to reach through traditional note-taking and textbooks. Instead of a diagram in a text book, a professor can show a life-size three-dimensional diagram of a biology dissection. Instead of trying to describe the ruins of ancient Rome, they can be shown on the big screen in high-definition video.
When teaching a class that involves the use of computers, having a projector allows a teacher to demonstrate how to perform a given task live, rather than trying to describe it. For instance, commerce students see how accounting software is used for certain functions. Beginning students are shown how to use basic research tools such as a library database. Computer science students perhaps see the most of all, as professors can demonstrate any concept on-screen.