To develop some opt-out feature in the future that will record audio as often as possible, activated by an update with this information hidden behind a long wall-of-text terms of service that nobody will read.
Nothing, I think it was hardware that was included because they might one day add a feature which used it, under the belief that customers would want that.
What I think is nefarious is not informing customers about that latent capability, because denying customers that information increased sales (eg, by not informing people who would be concerned by the mic).
A lie of omission for “good intentions” often is actually just denying the other person the information that they need to make an informed decision, for your own benefit.