It depends on what you intend to use the knowledge for.
If you want to enhance your career, going for a paid course could add credibility to your credentials.
Another reason why someone may go for a paid course (if it's nothing to do with career; probably personal development / interest) is, it accelerates your learning, shortens the steps you may need to go through, and in turn, save time.
Going on free courses may or may not extend the quality of your knowledge in the topic. Sometimes these are like teasers to paid courses that are more in-depth.
If the provider is in business for profit, why would the course be free?
Write your thoughts