A separate business bank account will help you to:
clearly and easily show your business finances separate from your personal finances
analyse your cash flow
monitor your business income and expenses
extract information needed to meet your tax and reporting obligations
easily get detailed records of your business transactions, particularly if you put those transactions through a limited number of accounts.
If you have a business credit card attached to your business bank account, and do most of your transactions through your credit card, your bank statements will, in effect, be a checklist of your expenses.