Look at it this way. I am currently looking at cars, and last month many dealers had 1% loans for 3 years. I am in Australia, and interest rates are a lot different here than they are in the US. 1% is amazing! I can get 3.75% on my money at my bank - I could get more if I put it in a term deposit for 3 years. So I could buy a car for $20,000 and hand over the cash. I could also buy a car for $20,000 and take out a loan and put my $20,000 in the bank (in a term deposit for the three years), and earn 2.75% (3.75 - 1) interest on the $20,000 for 3 years - so I would have an extra $1650 just by taking out the loan rather than paying in full!
Of course, this assumes that buying a car is a good thing to do, which is an entirely different discussion! Most MMMs would consider it $20,000 down the drain.
Note: this isn't really right, because I have left out monthly payments, simplified the interest calculations, assumed no extra charges on the loan etc.