Can I give an example to extend your comment.

I have a remote shop that takes cash and credit card payments. If i just ran a cash book like some on here are suggesting how would I know my sales for a day? I could wait until the money hits my bank account and categorise it, but then how much of that money is sales tax not sales? How much did the goods they sold cost me? Did they put all of the cash in the bank or did they steal some. Did the credit card merchant pay me everything due?

Imagine the customers have credit terms with you? Now when do you know your sales, when they pay? What if they don't pay?

Double entry has answers to all of those questions.