The two Silver Surcoats are part of the magical items bestowed by Flora and Schierke to Guts and his team to help Schierke in her fight against the Trolls raiding Enoch Village, along with Farnese's Silver Dagger, Serpico's Sylph Sword and Sylph Cloak, Isidro's Salamander Dagger and the refused Axe of the Gnomes.
The Silver Surcoats have no magical might on their own, but nevertheless, for the still weak and unable to use and understand magic Farnese, and the helpless Casca are meant to be the last line of protection, as silver is a metal with the ability to repel evil beings.
Farnese and Casca always wear them under their garments, with Farnese getting an additional silver item, a dagger, to defend herself and Casca by demonic creatures still attacking them despite the silver surcoats.
Before returning to Vandimion's household, Farnese sent back to Schierke her coat and dagger, as a sign of her intention to stay home and convince her rich father to give Guts a ship to search for Skellig island. However, they were returned to her when Frederick III allowed her daughter to go with the Black Swordsman.