Just about every day, I have someone looking at my screen from a relative distance (over my shoulder, 5 feet away, etc). This is because I'm showing them how to do something... impromptu training of sorts. I would think that most users are similar. The problem is, with high resolution screens, it can sometimes be hard for the other person to really see what you're showing them unless they have really good eye sight (and even then, if can be difficult).
Mac OS X has a really great (little-known) feature built-in to help in just this situation. It allows you to easily and quickly zoom into a portion of your screen, enlarging it for others to see more clearly.
To enable the feature, open System Preferences then click on Accessibility. From here, click on the "Zoom" category on the left, then check the box called "Use scroll gesture with modifier keys to zoom:" You can choose any of three possible modifiers to use the zoom feature. I have chosen to use the Control key.
With the feature enabled, let's try it! Hold down your modifier key (Control in my case) then use your mouse to scroll up and down. You'll see the screen zoom in and out based on where your mouse is located. You'll also find that as you move to the edge of your screen, the screen will follow your movements. This is a great feature for training or otherwise emphasizing a specific area on your screen.