Bleed...the printing term silly!
When something is designed for print and has a portion of the design inteded to go directly to the edge of the "page", the file needs to be setup to have a "bleed".
Essentially, the design (or graphic element, text, etc.) needs to "bleed off the page".
Well there are 2. Both reasons come down to their being shifts of the paper while being printed/handled.
First, it is merely to acommodate the minute shifting of the paper in the machine that periodically happens when the machine is running due to vibration. So if there is a shift, there can often be a thin line of inkless paper on any of the edges that are supposed to have ink there.
Second reason is due to periodic shifts when being put through the cutting machine. A bleed is to allow the stack of paper, that was just printed on, to shift a smidge without leaving a hairline of inkless media. Just like when shifting in the printer happens.