Custom Validators - When to use Promise.reject() vs throw new Error()?

In the Custom Validators/Sanitizers documentation, it offers 2 methods to return an error along with a message.

body('email').custom(value => {
  return User.findUserByEmail(value).then(user => {
    if (user) {
      return Promise.reject('E-mail already in use');
    }
  });

The following code also works:

body('email').custom(value => {
  return User.findUserByEmail(value).then(user => {
    if (user) {
      throw new Error('E-mail already in use');
    }
  });

I understand the Promise.reject() method is asynchronous, however it doesn't seem to make a difference in the actual code.

Is there a particular situation you would want to use Promise.reject() vs throw new Error()? Or are they just the same thing but different semantics? And which method should be prioritized in situations where both methods work?