In the UK there's such a thing as an "interest-only mortgage" where you don't pay down the principal at all! Which of course means lower monthly payments.
The theory is, if your mortgage interest rate was X% but some other investment like the stock market had much higher returns, a sophisticated investor might want to take the money they would have spent on repaying the principal and instead put it into that better-performing investment, then after 20 years pay off the mortgage principal with said investment.
Post-financial-crisis rules were put in place that mean banks actually have to ensure you're _making_ that higher-performing investment.
The theory is, if your mortgage interest rate was X% but some other investment like the stock market had much higher returns, a sophisticated investor might want to take the money they would have spent on repaying the principal and instead put it into that better-performing investment, then after 20 years pay off the mortgage principal with said investment.
Post-financial-crisis rules were put in place that mean banks actually have to ensure you're _making_ that higher-performing investment.