When objects are too close, the photo can cause eye strain and uncomfortable viewing. This concept is embodied in the 'stereo window' that is physically built into every stereo camera. Limiting the depth in stereo photos to ensure general viewability is the math we are talking about. You don't ever have to calculate it if you don't want to. But if you compose your stereo photos without any regard for the geometrical limitations of the medium, some of your slides will be hard to view, or cause some people headaches.
Some scenarios are more critical than others.
You don't have to drive at the posted speed limit, you merely shouldn't drive faster. You may want to shoot with much less depth for any number of valid creative or practical reasons. If your eyes can accomodate more depth, and you will not share your slides with others, you can shoot more depth. Many competent stereo photographers use more than one method, as the situation dictates. Playing with a spreadsheet will quickly familiarize you with the limitations of your apperatus.