Business money and personal money in the same accounts is called commingling, and it causes three separate problems.
The figures stop meaning anything
Every report is built from what went through the accounts. Personal spending inside them inflates costs and understates profit; business spending from a personal account does the reverse.
Neither is visible on the statement, so the figures look complete and are not — see real cash balance.
Tax becomes an argument
Deductions have to be substantiated. A mixed account means every transaction is potentially in question rather than only the unusual ones, and reconstructing which was which, a year later, is the work nobody has time for at filing.
Liability protection can be undermined
An entity protects personal assets because the business is a separate legal person — see business entity types.
Where the owner treats the two as one set of money, a creditor can argue the separation was never real and reach personal assets anyway. Commingling is among the most commonly cited grounds.
This applies to companies rather than sole proprietorships, where no separation exists to lose.
Taking money out is not the problem
An owner is entitled to take money from their business. The requirement is that it is recorded as what it is — an owner's draw or a distribution — rather than disguised as a business cost.
What keeps it clean
A business bank account and card used only for the business.
Personal costs paid from personal accounts, and where one goes through the business by mistake, marked as personal rather than categorised as an expense — see marking a transaction personal.
Owner money moved deliberately, on a schedule, rather than by taking business cards to personal purchases.