It's often framed as useless monkey business, rewriting working software to produce a copy of it but in another language, for questionable benefits.

I'd say that rewriting anything in any language (even in the same language) would remove large amounts of cruft, and add long-missing neat things that are easier to add when you build from scratch, and with a good understanding which the original authors lacked. Often it also can afford using a better architecture, see rg vs grep: grep has many brilliant technical solutions, but making it multithreaded would be a major rewrite anyway.