The main reason you would want to pick such a proprietary library is to achieve a homogeneous look and feel (often behavioral UX as well) when your app must work within another product - this product/organization is usually the one that also provides the library in question, and more often than not, uses the same design system if not the library itself.

We built https://github.com/freshworks/crayons for the same reason - apps published to the Freshworks Marketplace can be built using Crayons. We also ended up building our own user facing SaaS applications using Crayons.