An accurate lastmod date in your sitemap can hint Google to re-crawl a changed page, and Google does use it as a signal, but only as long as it stays honest and reflects real changes. The mechanism is conditional: a truthful lastmod that updates when you genuinely revise content helps Google prioritize what to revisit, while a lastmod that gets bumped without real changes gets distrusted and, over time, ignored. So this helps when truthful and quietly backfires when gamed.
The reason the honesty condition is built in is that Google has seen lastmod abused for years. Plenty of sites set the date to “today” on every URL automatically, or refresh it sitewide whenever anything trivial changes, hoping to trigger constant re-crawling. When the date repeatedly says a page changed and the crawler arrives to find nothing meaningfully different, the signal stops being useful. Google learns that a particular site’s lastmod is unreliable and discounts it accordingly, which means even your genuine updates lose the boost the field could have given.
Used honestly, lastmod earns its keep. When the date moves only because you actually edited the page, and it reflects a real, substantive change rather than a cosmetic tweak, it becomes a credible hint that this URL is worth revisiting sooner than its neighbors. That is what makes it valuable on a large site where crawl attention is spread thin: an accurate lastmod helps the crawler spend its time on the pages that truly changed. This is observed behavior worth confirming against your own crawl stats over time, since how heavily any single signal is weighted can shift.
To get the benefit, set lastmod to reflect real changes only. Update it when you meaningfully revise a page, leave it alone when nothing of substance changed, and resist any plugin or process that stamps the current date across your whole sitemap automatically. A lastmod field your crawler can trust is a small, durable advantage; one it has learned to ignore is worse than none.