In practice
Use it for crawl waste: faceted filters, internal search, session parameters, endless calendars.
Never use it to hide a page from results. A disallowed URL can still be indexed from links elsewhere, and because the bot cannot read it, it cannot see your noindex either.
Why it matters
It is the cheapest lever for protecting crawl budget on a large site.
It is also the most misused directive in SEO, and the damage — deindexed sections — is usually discovered weeks later.
Related terms