Warren Buffett, at the ripe age of 18, learned to grow his wealth in a way many young startups couldn’t. Buffet and a high school friend bought a pinball machine to put in a barbershop, hoping to make a few bucks. However, he didn’t run out and spend his earnings on new sneakers or a cool car: He reinvested them.

Buffet and his friend bought more pinball machines, eventually stocking eight local barbershops. When they sold the fledgling business, he used his profits to buy stocks and start another company. Today, Warren Buffet — who once was nothing more than a kid with an arcade game — is worth more than $67 billion.

Why should I reinvest?

Regular reinvestment has huge benefits for startups. Reinvesting increases working capital and leads ...