Pros and Cons of Paying Off a Loan and Not Paying off a Loan

[Michael Faudoa]

Hi all, my name is Michael and I am a new member here on BP. 

I have a question that I am hoping someone can answer- 

Why do some people advocate that you never pay a loan down because a loan is a wealth generator? I know that a benefit of paying off a loan is increased cash flow, but I am curious how not paying down a loan can also generate cash. 

In Brandon Turner's book "The Book on Rental Property Investing", he states that you have your tenants paying your mortgage for you, and that he could work a minimum wage job for the rest of his life and still retire a millionaire because of the tenants paying down the loan. I don't understand though?