Why is –isolatedModules error fixed by any import?

In a create-react-app typescript project, I tried to write this just to test some stuff quickly:

// experiment.test.ts
it('experiment', () => {
  console.log('test');
});

But it gives me the following error, with a red squiggly beneath it:

All files must be modules when the ‘–isolatedModules’ flag is provided.

However, if I change the file to the following, then everything apparently is fine (except for the unused import of course):

// experiment.test.ts
import { Component} from 'react'; // literally anything, don't even have to use it

it('test', () => {
  console.log('test');
});

Why? What is happening here? What does --isolatedModules actually mean/do?