Identifying guest posts as such is a matter of transparency and relevance to both readers and search engines like Google. Here are several reasons why it’s important:

1. Transparency for Readers: Readers expect to understand the context of the content they consume, including who wrote it and why. Labeling a post as a guest contribution clarifies that the content comes from an external source and may represent a different perspective or expertise.
2. Content Quality and Trust: Guest posts can vary in quality, and readers may scrutinize them differently than regular content. By identifying guest contributions, publishers maintain trust with their audience by being upfront about the source of the information.
3. Search Engine Optimization (SEO): Google’s guidelines emphasize the quality of content as a significant ranking factor. They advise against using guest posting solely as a means of link building. Identifying guest posts helps to differentiate between regular content and contributed content, which might be subject to different quality control measures.
4. Google’s Webmaster Guidelines: According to these guidelines, website owners should be cautious about guest posting for link-building purposes. Google advises against it.
5. Authoritative Backlink Profile: When guest posts are identified, and the author’s credibility is established, it can contribute positively to the site’s backlink profile, assuming the guest author has relevant expertise and authority in their field.

In summary, while Google has not explicitly stated that guest posts must be identified, the practice aligns with Google’s overall emphasis on content quality, authenticity, and user trust. It’s part of good webmaster practices and helps ensure that the SEO benefits of guest posting are realized without risking penalties.