i get what you are saying by pay for your purchases in cash when you buy them. you owe nothing after that for the items.

BUT you are bypassing the part where he says " I had the money to pay cash, but why use my money if I can use their money for free?"

he is basically saying he could have used his cash to pay in full if he wanted to. he chose to INVEST his $$$ to earn more $$$ and use somebody's else's $$$ for free to pay for his purchase them pay off the borrowed $$$ over time.
for a more THEORETICAL way to view this. in real life there are no guarantees. if he chooses to invest in the stock market, currently earning 10% annual return. he would be kissing $500 of earnings goodby by paying cash on his $5,000 furniture purchase.
if he gets sick or louses his job all he has to do is cash in his investment to pay off the bill.