We were in your shoes a year ago. There is (at least) a two fold reason for the hearing aids in the months leading up to surgery.
First, if the hearing tests were inconclusive or contradictory, there is a chance that you'll notice a good response from the hearing aids. If this is the case, the center would probably talk to you about amplification with aids rather than CI. An ethical center will not implant an infant who is receiving benefits from hearing aids.
More likely though, your tests were conclusive and you've done your research and are headed towards CI with certainty. If this is the case, the aids are worn to supply any possible stimulation to the otic nerve. It was explained to us that continued stimulation, however slight, would help to preserve the functionality of the nerve in the months leading up to surgery.
I agree with the others here that you should strongly consider starting some kind of signed language. It is important to know (and you probably do) that the CI isn't a fix-it switch, but rather another communication tool. Our daughter is 2 and has been using her CIs for one year now with excellent results- she can hear and comprehend the slightest whispers. Her speech production has been an added bonus, and she continues to use sign.
We look forward to hearing about your progress!