Proof that the applicant’s job is genuine and his income is legal. Proof of income is a required document when applying for a principal credit card. It provides convincing proof of income and helps the card issuer issue a high-limit credit card to you faster. The purpose of providing a work permit and business card is to prove that the applicant’s job is genuine and his income is legal. A credit card, also called a credit card, is a credit certificate issued by a commercial bank or credit card company to consumers with qualified credit. It takes the form of a card with the name of the card-issuing bank printed on the front.