Meta's use case are OTT applications on the Internet, which are usually billed per byte transmitted. Reducing the bitrate for the audio codec used lets people talk longer per month on the same data plan.
That said, returns are diminishing in that space due to the overhead of RTP, UDP and IP; see my other comment for details on that.
More than that, in developing countries, such as my own, Meta has peering agreements with telephony companies which allow said companies to offer basic plans where traffic to Meta applications (mostly whatsapp) is not billed. This would certainly reduce their costs immensely, considering that people use whatsapp as THE communications service.
That said, returns are diminishing in that space due to the overhead of RTP, UDP and IP; see my other comment for details on that.