What is Artwork Bleed?
Banner stand graphic being trimmed
If your background colour stops on the document edge and doesn't continue in the bleed area you may see a white edge after trimming. Imagine trimming the above example image to the outer crop, if your ruler was not 100% straight or you used too much pressure on the blade you may see a white edge.
Bleed basically makes trimming more accurate, ensuring your solid colour runs to the edge.