Hi Brian,

> Burrs method works, but why? There are two planes formed at the top,
> and two planes formed at the bottom.

It sounds like with your method you created 2 planar cap surfaces individually but if you create them separately like that by selecting the top and bottom curves to planarize then you will need to select all those 3 surface parts and use the Edit > Join command to glue together all those separate surfaces into a fully joined and closed solid.

Burr's method is a shortcut - the Planar command not only allows you to select curves as input to it, you can also select a surface object that has planar openings in it and when you give it that input it will construct planar surfaces for those openings and also automatically do the join for you as well. If you select curves as the input it does not know anything about the other surface piece and does not do any automatic join. So that's the difference.

- Michael