1) Even with print runs of a few thousand books per year, you can generate a very decent additional income ( German blogs all have thousands of readers and many earn good money).
2) Managers, doctors, coaches and specialists of all kinds don't even attach any importance to this: They are primarily concerned with the gain in reputation that a book always brings with it and which usually benefits their main business in a very concrete way (Worth reading: The 10 best marketing books that you should know)
3) For example, a study by guatemala telegram screenin Philipps University in Marburg in 2010, which examined and compared the effectiveness of various marketing measures by service providers (coaches, trainers, etc.), shows that one's own book is by far the most effective marketing measure. Far ahead of one's own homepage, social media/network, blogs , internet and word-of-mouth marketing. At the same time, this type of self-marketing is one of the least used. Writing your own specialist book for marketing purposes is therefore useful in two ways: it is highly effective and only a few people do it.
4) And of course there are still a large number of 'conviction-driven writers' who want to write their own book primarily for themselves. Be it an autobiography, a novel , a collection of stories, a children's book, etc. For them, it's primarily about writing and the desire for their book to be published. Marketing, sales figures and royalties are not very important to them.