That does sound strange and I don't know enough about fibroids to understand why that might be. Zoladex turns off all your hormone production and stops your FSH and LH as well - it's also known as "chemical menopause". Many people then have add back HRT as you've mentioned in order to prevent menopausal symptoms (though btw most experts now don't recommend Tibolone as there are better options available, ie. transdermal and Utrogestan).
But as you rightly say, if that's what you're taking anyway then I'm not sure what the benefit of the Zoladex part is. Many doctors are recommending Zoladex to me for different reasons, but the big difference is that I am still ovulating and I'm assuming from your post that you're not? That would be the only thing I could see that made sense - that if you weren't yet post Meno, that the Zoladex would switch your ovulation off and that would be their intention. If however you're post meno, I'm not sure I understand the logic at all!