My understanding is that sometimes if your ovaries do not function well, you may not release enough progesterone to support the luteal phase of your cycle and/or early pregnancy, even if the embryo is technically viable, so there may be value in progesterone support in that case.
I don't know that there's any harm to adding some in just in case...?
Yes, in cases of true progesterone deficiency it does help. Unfortunately I have found that the medical profession debates like crazy over the treatment of progestrone, it caused me considerable confusion and hurt when two well educated medical professionals disagreed over the best course of action for me. One said it would do nothing, the other said it was the most effective treatment. One said it can cause gential defects in boys, the other said there was no harm in it at all. One said that my progesterone levels had increased to a safe level, the other said that only the blood progesterone increased and not the progesterone produced from the CL as required. It drove me mental. In the end I took the progesterone and was glad I took it.

If you are really worried I think you can buy some progesterone cream over the counter at the Chemist.

I can really understand your concern. Please let us know how you go and don't let the debate get to you, but be prepared for more Doctors to say as your GP did.