The guide on setting user agents in Node.js emphasizes the importance of the User-Agent header, which identifies the client software making an HTTP request and can help distinguish between genuine users and automated bots. The default user agent in Node.js is simply "node," which may trigger anti-bot systems, making it crucial to customize this header when performing HTTP requests. The guide explains how to set a user agent both locally and globally using the Fetch API, and discusses the benefits of user agent rotation to avoid detection by anti-bot systems. It also highlights the limitations of these methods against advanced detection mechanisms and suggests using a Web Scraper API for more robust solutions. The guide is authored by Antonello Zanini, a technical writer and software engineer experienced in web development and technical content strategy.