Once every few days of using the Vault app with your Vault Signer Card, the 'Signer Card verification' screen will be displayed asking you to scan the card to continue using the app.
Stellar uses signatures as authorization. When you've used your Signer Card to sign in to your Vault account, you've used your signer account to sign the special "challenge" transaction that identified you as an owner of this card and authorized you to use the Vault application for a certain time period.
Your signer account is stored on the secure chip inside the Signer Card and is never exposed to any software, including LOBSTR Vault.
Therefore, you'll need to physically confirm that you are still in possession of the card and the account by scanning the card every time the Vault app prompts for this action.
Scanning the card authorizes you to use the Vault app further and acts as a security measure by verifying your possession of the Signer Card and the signer account.