The computer screen is a lot wider than it is tall. The computer image doesn't exactly fit on the TV screen. The class only sees the last image frozen. The teacher or their assistant can work on their computer preparing for the next image without disturbing or confusing the class. This can be very handy when the teacher wants to look up some obscure reference or check some definition.
Some units are capable of a few thousand colors and others can show millions of colors.
Be careful when interpreting such information however, your converter won't be any more powerful than your computer. I once proudly marched into Bible class with a notebook computer and PC to TV converter to display the beautiful million color images that I had processed on my home desktop computer. It was more than a little disappointing to find out that the notebook computer I was using was only set up to display 16 colors. It was funny at the time but sure took a lot of punch out of my presentation. No image system can project any better image than the software or computer's video card is capable of showing.