Before verdaccio, we used to post tarballs that customers would download and use, but that became cumbersome. We looked into the official npmjs service, but their pricing model ($7/user/mo, sharing private modules requires all users to have a paid account) didn't make sense in a situation with many users downloading once a month. Verdaccio let us manage the distribution on a single $5/mo digital ocean droplet and it's been working very well. We post modules to a separate scope and users configure their npm clients to download from our registry for that specific scope.
It was the first time we came across an open source project where we could actually quantify the saving, and our contributions through their OC (https://opencollective.com/verdaccio) are a drop in the bucket compared to the savings.