The mast that your phone communicates with doesn't always have lots of fiber bandwidth. Sometimes it's only connected to other masts using microwave links (which could be outdated).
https://web.whatsapp.com/ only allows login through a cert (via a QR-code). Some banks also use this. A smartphone is used a a cert vault. Not a client cert in the traditional sense, though it's basically the same thing.
What it reminds me of, is Google Project Tango, which also has the NASA's JPL listed as a partner[1]. Also worth mentioning, is Johnny Lee, who worked for Microsoft Kinect, and is now working at Google for project Tango.
from http://www.reddit.com/r/netsec/comments/209h4d/samsung_galax... :
This is not a backdoor. It's a feature, and a reasonably common one for Qualcomm based devices.
It's an interface to allow the modem access to a persistent data store (ie. eMMC modem partitions) even though only the application processor may access the MMC controller.
Have a look at the rmt_storage client documentation found in a Qualcomm kernel tree. It used to be pretty common to ship a rmt_storage daemon to do the very same thing Samsung is being accused of here (hint: Nexus 5 still uses it), I don't know about other recent devices, but I'd imagine they'd employ something similar.
Also, there are many more ways for the baseband to compromise the application processor, without an explicit interface.