A support contract, typically. Few people sell code alone. They sell code plus a contract to support that code.

Support contracts can be very lucrative.

It's usually included when you purchase a commercial license.

I work with a company that generally stays away from open source software, unless they have an in-house developer that can maintain it.

Most open source projects (unless it's backed by a huge company that also charges for support) are run by volunteers that have a day job. This means that support will be non-existent beyond what the developer(s) feel like and bug fixes/updates may or may not happen. This doesn't work that well when a company relies on this software for any critical task.