You can trade them and seem to be somebody else, cover them and get into fights anonymously, or, if you are a damsel, show up at court with one and ask for a knight to finish the dead original owner's quest. You can make veiled innuendoes on shields which you give knights to carry unsuspectingly.
Lancelot left his own shield with Elaine of Astolat while he borrowed her brother's; she nurtured her love for Lancelot while caring for his shield, found out from Gawaine (who recognized the device) the identity of her hero, and eventually died of love. A shield can even be used as an offensive weapon to finish a fight successfully after your sword has been lost or broken. The uses of shields are limited only by imagination.
Not infrequently, knights refer to having recieved their shields as gifts from their ladies. A young knight bore a blank shield until he had earned the right to a coat of arms.
Escu | The Legend of King Arthur