The separation reason is the strongest. A single card used only for business expenses means your accounting software receives a clean feed of transactions to categorise rather than a mixed one requiring you to sort personal from business every month. That is hours recovered at tax time and, more importantly, deductions you can actually substantiate. Mixed spending on a personal card is the most common cause of a first year business being unable to claim things it legitimately spent.
The second reason is that it builds a credit history in the business name, separate from yours. That history is what makes future borrowing, supplier terms, and equipment financing available on the business rather than requiring you to guarantee everything personally. It takes time to establish, which is the argument for starting before you need it rather than when you do.
Be clear about what it does not do initially. Most cards for new businesses require a personal guarantee, meaning you remain liable if the business cannot pay. That is normal and it does not undermine the separation for accounting or liability purposes, and it does mean the card is not a way to take on risk the business alone carries. Read that term rather than assuming the business name on the card changes who is responsible.
Choose on the ordinary criteria rather than on rewards. Whether it reports to the business credit bureaus, which not all do and which is the whole point of building history. Whether it integrates with your accounting software so transactions arrive automatically. What the annual fee is against what you will actually spend. Rewards on a first year business volume are worth very little compared with any of those.
Then use it for business only, without exception, including the small things. A card that is ninety percent business is a card somebody has to audit, and the discipline is easier to maintain absolutely than approximately. If you accidentally use it personally, record the repayment as an owner draw immediately rather than leaving it unexplained, which keeps the record defensible.
Set up a payment arrangement that removes the possibility of missing a due date, because a late payment on a business card damages exactly the credit history you opened it to build. Automatic payment of at least the minimum, with a calendar reminder to pay the balance in full, covers both the reporting risk and the interest. Carrying a balance on a business card is expensive money, and for a first year business it is usually a signal to look at cash flow rather than at financing.