When would you choose not to market your book? When there’s no market for it.
“If there’s a book inside you demanding to get out, write it,” she said. (I’m paraphrasing; apologies for any misquoting.) “But if you don’t think there’s a market for it, then don’t market it.”
If you think your book would be of interest only to a small group of friends and family, just have a few copies made up by one of the Print on Demand publishers and give them away as birthday or holiday presents. This will cost you a few hundred dollars, no more than you’d spend on other gifts. They get a great personalized gift and you get to see your book in print.
Marketing a book, like any other marketing, is an ongoing effort. Even though there are “easy ways” to do that marketing, it takes time and at least some money. If you don’t expect your book to repay you with either direct or indirect income to compensate for that effort, then don’t market it.