I've generally assumed companies do this to lock devs into their ecosystem. In theory you may get situations where a company does something so incredibly specific that they need a DSL, but even then from a dev's point of view I'd prefer a really great framework in a more popular language.
The purpose should be to enable fast, high-quality development in the ecosystem. So if the proprietary language is really easy to learn and makes it really easy to get things done... fair enough. Those are some pretty big "if"s though.