People have speculated that this is an attempt by some attacker to correlate ownership of older and newer addresses en masse. Some wallets give the user no control over which address(es) are used as the source for any given payment. When a tiny sum shows up in some old empty address still being tracked by the wallet, those coins will eventually be sent on automatically as part of a larger payment, alongside coins from one of the user's currently active addresses, and suddenly the common ownership of those two addresses becomes public record.