Cplanes and Commandscripting using the Entablature profile as an example.

There are two ways to enter coordinate points in Rhino. One is in world coordinates and one is in cplane coordinates. I have been using world coordinates mainly because this makes a command script work predictable under almost all circumstances.

You can move and designate the cplane coordinates in a script. Than you will be able to use cplane coordinates that act predictably under almost all circumstances too. This is of some help but mostly you can stick with world coordinates for scripts that you want to be universal. Otherwise there is no way of knowing where a fellow user has their cplane origin at and thus your objects or scripted procedures will end up in the wrong places and probably will not work. You can specify a cplane origin but than by that time it is also easy to use world coordinates. So why use the cplane in scripts? The biggest reason is that you can make many of your scripts user interactive and more universal for yourself to use.
! Circle
This creates a circle for the Entablature profile to sweep around.
This is the world x,y,z coordinate center point of the circle.

Here is what I mean. Take the script above, notice where the circle and thus the Entablature profile is located. It is specified in world coordinates. This is ok when you want to share a script or use that same location. It is also not too hard to change the locations' coordinates if you relocate the Entablature profile somewhere else in 3d space, but it is bit of hassle. Here is where using the cplane coordinates instead of world coordinates comes to the rescue. 

If you remove the world location and enter the radius of the circle which is the first location point and 0 the commandscript now reads... 
! Circle
! selnone
This creates a circle for the Entablature profile to sweep around.
This is the cplane x,y,z coordinate center point of the circle.
Tells Rhino to draw the circle using radius.
This is cplane x,y,z coordinate of the radius of the circle.
This selects both curves.
This sweeps the entablature around the circle to create a 3d surface.
This completes the command.
This de-highlights the curves that were used to create the surface.

1. Highlight and copy the left-hand column of script above
2. Back in Rhino move the Entablature anywhere in space

3. Click the Set cplane origin world top icon  

4. Click the endpoint shown on the diagram above by using snap to endpoint.

5. Make sure the perspective viewport is active and click the Run Commandscript icon

This is how the Cplane is useful it lets the you the user choose where command scripts take place. By you picking the origin so you don't have to cut and paste world coordinates all the time. This is great for one user only remember scripts that you want to share with others you should be careful with user input of the cplane, you really can't trust things to run smoothly. But as you see these are extremely useful for your everyday modeling routines.