Definition
An XML sitemap is a file listing the URLs on a site, with optional metadata like last-modified date and change frequency, submitted to search engines so they can discover and index pages efficiently, especially ones not well linked internally.
Why it matters
On a site with many programmatically generated pages, the sitemap is how a crawler discovers them all. A missing or incomplete sitemap means pages that exist but never get indexed, which is traffic left on the floor.
How we think about it
Every site we build generates a complete sitemap from its data, split into sub-sitemaps when the page count is large, and references it in robots.txt. The sitemap is generated, never hand-maintained, so it never drifts out of sync with the pages.