Because high utilization tells banks or lenders that you’re probably getting way over what you can pay with your income and should probably not lend you more money that you might not be able to pay.

The recommendation to use credit cards is mostly so if the card gets swipe with fraudulent transactions that they criminals didn’t spend your own money and therefore it’s much easier to dispute, much easier for them to close the account and then re-issue you a new number / card. And also there are rewards to take advantage off.

You should be using credit card like a debt card. Spend what you can pay / afford monthly and pay it off.