You can use your marketing skills to show business owners the benefits of using analytics data, strategic keywords and content structure to gain more organic web traffic. If you are unfamiliar with SEO or want to brush up on your digital marketing skills, you can reference Moz's Beginner's Guide to SEO. Keep in mind that Google's algorithms are always changing, so SEO is something you will need to continue your education on to stay relevant and successful in this field.
I wouldn’t say you can’t be successful with a free domain, for example barenakedscam.siterubix.com, but having a personal domain will definitely help with branding and makes it more credible, plus there’s no restriction on the design if its your own domain. Free domains have certain restrictions in terms of the customization of the look and feel of the website.
In the long-term, it affects your versatility, functionality, and, of course, your brand. In the short term, it can certainly add/take away a lot of headaches. That said, just like choosing a physical house or office, there is no such thing as an absolute “best” or “top” choice. There’s only the right choice relative to your goals, experience, and circumstances.