Chalk this up to general principle and not exact science, but it basically works like this. I can buy an $18k car using a car loan. I put down $1500 of my own cast and take out a loan for the rest ($16.5k). With a 5% interest rate I'll end up spending just over $19k for a $16.5k original loan value. Or, I can take that $1500 initial investment and put it into a mutual fund. I can then fund it monthly at the same amount I would have paid on the car loan (~$263 per month) over 6 years, and even with the same 5% interest rate, which is rather low for a mutual fund, I'll end up with about $24.5k in the end. So I would have spent about $19k for $24.5k in total value.
Obviously, rates of return and interest rates and market fluctuations and all that jazz vary, but you get the gist. I can fund a mutual fund for a few years, earn money on it, pay my taxes on the gains, pay for the car outright, and still have cash leftover. Or, I can take a loan out and lose money throughout the whole process. And yes, I can lose money in a mutual fund too, but would you rather put your money into something that's always, always, always going to be a loss, or would you rather put your money into something that's almost always going to be a gain? Heck, even putting the money into a savings account would still save me money, because I would fund that same $19k for an $18k car.
I realize this is a very, very generalized viewpoint of it all, but it works, and the key to all of it is just being patient. Put enough money into a mutual fund over a long enough time, and suddenly you'll find yourself able to pay for new cars, new HVAC units, new roofs, new kitchen remodels, and all those major shiny new toy purchases through capital gains alone. The banks know exactly what they're doing by offering you loan terms on that mattress you just bought.