One of the most common uses of rewrites is to automatically redirect a URL from an old platform to your new site. If you've ever changed your platform (for example, moved from a proprietary system to WordPress or similar), then you'll have a large amount of existing links which will already be bookmarked and ranked within Google.
Like our 404 recipe, many modern CMSes can do this, but most can't do it with the efficiency that NGINX can. Anything which can reduce the server load on a production system is always a good thing, especially if you have a very busy site.