What’s the main use case for it? Prevent tracking when you don’t need to use it?
Discussion
Yes, prevent tracking of not removable battery phones,
isolate a secondary one for privacy purpose,
making credit cards and all RFID bages not readable from close( so you don't risk them being copy or leaking personal data to near possible scan)
Prevent harm done by strong radio and magnetic field to devices.
Never thought about the card thing, good idea.
Setup a new smartphone from scratch at a public wifi spot, not linked to your identity and no sim. At some point, you need to bring it back home surrounded with all the wifi signals, cell towers, bluetooth ... Google and Apple can easily guess who you are and switching the phone off is not secure. That's why you use a faraday bag when going back to you home location so that you do not leak anything related to your identity (location).
Nice. Thanks.