The Last-Modified Header is part of an HTTP response that contains the last time that a page was modified on the server.
This can be valuable to webmasters as it will tell them when the last time their webpages were updated, helping them keep their content fresh.
Last Modified Data Header & It’s Importance To Search Engine Optimization
The last modified header does carry some importance to search engine optimization, despite not being discussed much.
The freshness of content is a factor in SEO, and while not all pages contain a last modified header, they can be good for keeping webmasters honest about updating their content reguarly.
They also are beneficial for configuring your XML Sitemaps, as there is a field that reflects the last modified date, and a setting that is input to say how often the content might be updated.