The L-Plate makes it much easier to connect your camera to a tripod using either a horizontal or vertical orientation.
Before I owned an L-Plate, I hesitated to try to shoot vertically as the camera would inevitably shift positions when I shifted the ballhead to accommodate the vertical orientation. Now, it's a piece of cake!
L-Brackets (plates) mount on the camera and are then used to mount the camera to your tripod. However, to use the L-Bracket you must have an Arca-Swiss type head on your tripod because these are the ones that the L-Bracket attaches to. They are most commonly made by Really Right Stuff and Kirk Photo.
The L-Bracket allows you to mount the camera to the tripod in either Portrait or Landscape orientation and do so securely. Without the L-Bracket you need to tilt the tripod head to the side (not steady or possibly not secure) in order to shoot in the portrait orientation. The L-Bracket eliminates this problem.
If you are going to use a tripod these are very useful and recommended.
Thank you both very much for the speedy reply. I've been using a Slik tripod that raises to give me both angles, it's served me well up until now. However I do see eventual problems with the weight being off center when I eventually start using long lenses.