What makes it ok for companies to charge by the hour for work done

I was having this thought. If you have a job, you get paid whatever wage and it’s the company’s problem to pay that wage. You take what they give you, what you don’t give back is their problem. <br><br>But for a lot of services, companies charge the customer by the hour for work done. As a customer, you don’t get to ask how much their workers are paid. You have to pay the hourly rate they set at company level. <br><br>So why is it ok to charge the customer by the hour? Why is this an accepted model instead of a salary based model?