Why would you continue to add more luggage tags when there are perfectly serviceable ones still attached?

Or, if they are no longer valid/serviceable/contain out-of-date contact details, why would you not remove old luggage tags?

(I've also wondered - why do some people not remove the airline-applied luggage labels? I saw a woman last month at Schiphol whose entire case was covered with tens of those small luggage label strips that they peel off the main label and attach to various parts of the case. Defeats the purpose of having such strips, when no-one then bothers to peel off the old ones)