Contrary to popular belief it is very difficult to print to the edge of the paper. Many home printers give you the option of ‘borderless’ printing; however, professional printers don’t always offer this option.
“Bleed is a printing term that refers to printing that goes beyond the edge of the sheet before trimming. In other words, the bleed is the area to be trimmed off. The bleed is the part on the side of a document that gives the printer a small amount of space to account for movement of the paper, and design inconsistencies. Artwork and background colors can extend into the bleed area and after trimming, the bleed ensures that no unprinted edges occur in the final trimmed document.” – Wikipedia