Google has confirmed that a study using XML Sitemaps will;
help search engines find new and changed content faster.
Two important points can be taken directly from this study;
We conduct this test over a dataset consisting of over five billion URLs that were seen by both systems. According to the most recent statistics at the time of the writing, 78% of these URLs were seen by Sitemaps first, compared to 22% that were seen through Discovery first.
And more importantly;
The main observation from this study is that for an archival domain, Discovery is 63% “efficient” and Sitemaps is 99% efficient in crawling the domain.
Since creating an XML Sitemap is so easy and effortless, there is no reason not to have one.