Bill Perez reminds us of how regions can help us in our 3D design work.
"Region is a command that is underutilized by users who are creating solid models. With Region, you can quickly select the closed-loop 'sketch,' converting it to a dimensionless flat solid shape that you can extrude, intersect, or revolve into a more complex shape. Closing the loop with Region is faster than using Pedit to perform other conversions of sketches, and wastes little time. (This command is not as fast as PressPull, but to users who don't have the Select When No Command is Active setting turned off, it is faster than changing the setting, and then using PressPull.)"
Notes from Cadalyst Tip Patrol: A region is a two-dimensional enclosed area that has physical properties such as centroids or centers of mass. Regions are two-dimensional, but you can apply 3D editing tools to them; they are easily extruded and manipulated in three dimensions. You can do more with regions in 3D than you can with closed polylines. In addition to helping with 3D objects, regions are also useful in 2D drawings to create hatch areas or for shading.