A platform offers major advantage when it comes to selling books. Before social media, non-fiction authors had an edge. These authors already had an existing audience by the time their books were ready for sale.
Novelists, conversely, found themselves relying on a lot of pure luck, prayer, and alignment of the stars. The fiction author had little to no control regarding the business side of their business. The only way to build a platform was to not completely FAIL with book one.
Non-fiction authors, however, were not nearly as vulnerable because they had ways to cultivate a following ahead of time. Those ways also permitted them to KEEP growing the platform even bigger as they continued to publish more works.
For instance, if one happened to be an expert of some sort, it was far easier to build an audience interested in your topic. Therapists, psychiatrists, physicians, personal trainers, business owners, etc. obviously could begin with their ‘job’ (I.e. a private practice). Then these experts progressively expanded their platforms in a logical fashion.
They might broaden to speaking engagements, guest appearances on television and/or radio, serve as ‘experts’, and maybe even fold in lectures and seminars. With every expansion, the NF author added more numbers to their ‘platform.’