@babel/register is useful during testing, as it allows you to write ESNext inside your tests, as they will be transpiled down before the tests are run.
The alternative is to transpile manually using the babel CLI, and perform the tests on the transpiled code. This is acceptable; however, the line numbers on the transpiled code will not match the ones in the source code, making it harder to identify the failing test(s). Furthermore, since there is likely to be more boilerplate code in the transpiled code, the test coverage statistics might not be accurate.
Therefore, using @babel/register is recommended for running tests written in ES6.
