These services can host your content on their servers free of charge, but in exchange for that zero cost, your online destination will have a less-than-elegant domain, such as jeffreylwilson.tumblr.com. That might be fine for a personal blog, but it will look too low-rent for a business that wants people to trust it enough to pay for whatever it's selling.
This is a massive plus of website builders compared to web developers. A web developer can cost between $300 and $1,000 for template customization alone, and a fully customized website built from scratch will often amount to thousands. Making your own website using a builder, meanwhile, allows you to get a fully functioning website up and running for less than the price of a coffee.