Sitemaps are usually treated as a box to tick. Their most useful property is diagnostic, and almost nobody uses them that way.
The submitted-versus-indexed ratio is the signal
Search Console reports how many submitted URLs were indexed. A large gap is a direct measurement of content the engine chose not to store — usually a quality problem, occasionally a technical one, and either way a number no other report gives you as cleanly.
Split sitemaps to isolate the problem
Separate sitemaps per template or section turn one unusable ratio into a diagnosis. If product pages index at ninety-five per cent and blog posts at forty, you know precisely where to look. This is the single most useful thing you can do with sitemaps on a large site.
Do not list what you do not want indexed
A sitemap is a set of recommendations. Including noindexed, redirecting or canonicalised URLs sends contradictory signals and degrades trust in your lastmod values generally.
Honest lastmod values or none at all
A sitemap claiming every page changed today is misleading, and crawlers learn to discount lastmod from sites that do it. Accurate dates genuinely influence recrawl priority; invented ones remove that lever permanently.