Are you printing photos or not?
Bleed means printing all the way to the edge of the page
No bleed means a guaranteed margin on all sides
If you're printing photos that you want to cover the whole page, you need bleed. I use this for full page cookbook photos. The lack of whitespace gives cookbooks and picture books a more professional look.
If your book is text-only, you don't need any bleed. If your text has photos, bleed is a good idea.