For you, the buy and hold investor, “cash-on-cash”, as a measure of the return, is your true measure of performance.
For example, your rental house of 300k that you own outright, house that you net $15k in annual income, the coc return is 5%.
People use leverage, aka a loan, for more than one reason but one of them is to increase their return on their cash.
Play with different scenarios that you’re comfortable with in a real estate-related spreadsheet or calculator (meaning the amount of leverage) to see how the returns are different. In other words, see what cashing out 50% from the rental and putting it into another property looks like to see what the individual and combined returns are. All things being equal (property’s expenses and rents), you will see that your returns are greater with leverage. That’s why people use leverage and why few people simply pay their mortgages down. In fact, most pull money out at the right time to put that equity to use in another property.
Also, today’s money is worth less than next year’s money when there’s inflation. That means you’ll be paying back a loan with the future’s devalued money. And, don’t forget, all of the interest is deductible, something you don’t have with an all cash, 100% owned property.