I do this frequently. To prevent vendor lock in and allow us to easily pivot if pricing gets out line. We pay to support the project and get technical support when needed. Considering how little we use technical support. It should be a good deal for the company.

For one: Using open source version often is a lot simpler. Commercial versions are hidden behind authentication and other weird systems to download. User experience can be a lot better.

Then there are ideological reasons: Purposly trying to make the open source version sustainable.

And then reduced lockin etc. by not using Enterprise only features by accident/convenience, which leaves the door open to leave the contract.

Because I want to give a project money but also want to make 5000% sure the entire thing is in github, working, the latest, compiling and that we can do all of that all of the time? What is strange about that?

In my experience, you start using the open source version, realize you could benefit from paid support, so you "buy a license" and get your support -- but then you never have a big enough reason to do the lift to the commercial version.