Use it before every migration
Site migrations lose traffic when old URLs disappear without redirects. Compare the old sitemap with the new one: every removed URL should either redirect to its closest new page or be gone on purpose.
Comparisons ignore differences in trailing slashes only when you ask; otherwise /page and /page/ count as different URLs, as they do to search engines.
Questions
Can I compare different domains?
Yes. Turn on compare by path to match URLs across domains, such as staging and production.
Is there a size limit?
Up to 50,000 URLs per side.
What is the limit for one sitemap?
50,000 URLs and 50 MB uncompressed. Larger sites split into several files listed in a sitemap index.
Is my data stored?
No. Sitemaps are read or built for the result you see and not kept.