Companies pay for features
Being in the core team of a popular Open Source project will lead you to getting requests to do development for compensation within the realms of that project. You will learn that companies are at times willing to pay you to get their desired features added in your project so that they can use it – and they know that paying an already established core developer is possibly the best way to get a feature into a project's mainline code.
You will however rarely see a company offer to help with compensation for generic bugfixes or other infrastructure things or improvements in the project. And if you do, you know you are not in a small project anymore!