Borders are beneficial for you to include in your MOCs as they can help create a professional and finished look and add detail or aesthetics to the finished model. Builders can also use border to distinguish their work, as you can clearly tell from the 2nd and 3rd that they are by the same builder, thanks to the black boarder.
Borders can be any shape but tend to be square or rectangular, fitting to the shape of the scene or diorama. This can be done using SNOT pieces to position tiles on their side to create a smart look, or textured pieces such as grills to add further detail.
Some builders, such as Flickr user Inthert, gives some of his MOCs a circular boarder. This is done by using brick bending, as the 2x1 slope piece are positioned with a strain on them to bend into a circle, This looks very good and also cleverly covers the SNOT bricks he has used for the ground.