First off, before messing with isometrics, you should know what isometry is. Isometry is equality within measurement. What this means, is that in any isometric representation, all measurements are to scale, no matter how far close or how far in the distance they are in view. In an isometric perspective, you have a 3D view where, no matter where you are in that space, the object scaling retains its value and doesn't change.

Conversely, in a true perspective, objects reduce in size as they recede into the background.

Because of this lack of perspective scaling in isometric perspectives, they provide a way to represent a 3D object in 3D space while maintaining proportions and hence making it reliably measurable (for the most part). The following is an isometric cube that has all of its edges equal in length - represented in a 3D manner without true perspective distortion.

This can be invaluable in technical drawings for engineers and architects where an object can be represented, and therefore more easily comprehended, in a visually understandable manner but still maintains correct line measurements for referencing. For instance, in 4-panel technical object drawings (for engineering), much like the 4 views you see in today's 3D modeling software, you often have a front, top, side and isometric view (though in 3D apps your isometric is a scaled perspective).

I would whip out some of my blueprints to show this but they're buried deep in my closet and I don't know where my digital camera is. The isometric view is "to scale" (for the most part) and easily drawn because it relies on known measurements. This prevents the need for the draftsperson to have to calculate and determine perspective values for the 3D view, making it much easier and faster to produce. The ease of creation is the true advantage to isometric perspectives in that instance.

So far I've said "(for the most part)" twice already. What I mean by this is that not all dimensions in isometric views are actually to scale. What are to scale are the lines which run parallel to your x, y or z axes. Because these lines are skewed and at unusual angles, there will be distortions in diagonals and other lines of the such. This really isn't all that important though, and nothing to concern yourself about.

Senocular