Error Suppression with '@'

Good Morning, I was having a discussion with a fellow PHP Developer this morning and he mentioned that he put's an '@' sign in front of all function calls, and every time he accesses an array; I know that this is sloppy, and dangerous, but I don't know exactly what this exposes him to, can any one give me any real world examples of why this is bad, so I can relate it to his code? php.net does not have much information about this. It seems like suppressing errors, rather then catching them is problematic. Thanks Again. Justin.