Whether you like it or not, the need to promote your book is inescapable if you're going to make sales. That being said, not every opportunity to promote a book is equal in terms of the resources you have to put in or the results you're likely to get from investing those resources. Some "opportunities," to be blunt, just aren't worth it.
Take book fairs and festivals. What could go wrong? The fees for participation usually aren't exorbitant, and the whole purpose of the event is to put you and your books together with a large group of potential buyers and readers---many times the number of people who would normally visit even a large bookstore in a single day. In addition, you'll have opportunities to network with other writers and with professionals in the book industry. So, why not go to every one you can reach?
The answer is simple: not every festival will offer the same bang for the buck regarding your book or books. Some festivals are slanted heavily toward children's books, or toward certain genres; if your book isn't a good fit for the market the festival is best known for, you may spend all day at your table to sell only two or three books. Some simply aren't in the right location to attract the kind of audience that might want to buy your book; a book on some aspect of horse racing that would be attractive in Lexington, Kentucky, might be a much harder sell in Miami, Florida, or even in Louisville, Kentucky. Some festivals do a better job of attracting people who want to sell you their marketing expertise, writing classes, seminars, etc., than they do in attracting the audience you want to make contact with.
What can be said of book festivals can be said of other promotional opportunities that involve personal appearances. There is always a tradeoff between the potential benefit and the amount of time and money you'll have to sink into making the appearance, and that tradeoff can be harder to justify if significant travel is involved.
Opportunities that don't involve in-person experiences are generally less expensive, and as a rule of thumb, I'd say that if you have an opportunity to be interviewed by phone or Zoom for a piece in a periodical or website, or you have a chance to "appear" as a guest on someone's podcast or YouTube channel, go for it unless something about the party offering the invitation raises a red flag in your mind. These things typically cost nothing but your time, and most periodicals, podcasters, and YouTubers aren't going to waste their time on talking to someone they don't feel will appeal to their audience. That's a win-win; you get publicity, and they get an interview that they have reason to believe will help grow their audience. I'd be much more cautious with people who want you to pay for whatever their writing-related service may be. Exercise due diligence with these and check around with people you trust, because people offering paid services range from excellent professionals to slapdash or overly optimistic amateurs to scammers. (A good clue: if results are "absolutely guaranteed," take a long and skeptical look at more than the testimonials such services often rely on to sell themselves---even the best agents, editors, instructors, and marketers out there can do no more than improve your chances of success, and the good ones will be honest enough to admit that although they're confident in in their ability to provide value with service, they cannot promise great success every time.)
Promotion is important, but don't forget that there is an opportunity cost attached to the time and money you spend on marketing your book. Unless you're in a "one and done" situation as far as writing goes, time and effort you spend on marketing is time and effort you can't spend on getting your next book written, and over the long haul, creating a sizable body of work to keep engaging older fans and bringing in new ones is at least as important if not more so than relatively short-term marketing endeavors. Keeping an eye on both the forest and the trees is the needed strategy in developing a successful writing career.