The original idea of the field was to keep a hash of the contents of your wallet that you could update in the Ripple ledger. Your wallet would also contain any prior hashes of earlier versions of the wallet that you couldn't confirm had already been replaced in the ledger.
That way, a malicious (or negligent) wallet provider couldn't serve you an old/obsolete version of your wallet. You just retrieve your
WalletLocator from the network and if that isn't either the hash of your wallet or a prior hash stored in your wallet, you know you have a newer wallet out there somewhere. This would also catch the case where you store your wallet in more than one place and retrieve it from the "wrong" place.
Since the field was added, a different wallet model was used. But the field is still in the server and Ripple network in case anyone wants to use it.