If the post is from banks or similar, you don't really want the sender to be unaware that the person they're sending post to is no longer present at your address - as I understand it, credit blacklisting is sometimes applied to an address rather than a person, so if they think that an account which is overdrawn is based at your address, it can affect your credit rating.

Of course, banks won't necessarily believe "not present at this address" returned mail in this situation, anyway.