Well, it's extremely unlikely for a piece of code (or things like that) to be there by accident and still be useful to its user. (This is more or less why [HPMOR spoiler] Uneel fcrphyngrq gung zntvp jnf vagryyvtragyl qrfvtarq ol Ngynagrnaf.) This doesn't seem to generalize. (Think of “the purpose of X” as “the reason not to get rid of X”. In realistic situations, a given piece of code either has been placed there deliberately by someone or there's no good reason to keep it. This doesn't obviously apply to most other things, including life.)