Because some lawyer convinced a judge that there may be pertinent information on it. If there's any suspicion that you may have used your personal device to communicate with co-workers, then they'll want to see it. You can't prove you didn't without turning it over. If they find you didn't use it, then thanks. If they find you did, game over. That's the point of the discovery process. They don't know what information is out there, so they're going to get as much of it as they can and sort it out later.

> If there's any suspicion that you may have used your personal device to communicate with co-workers, then they'll want to see it.

No, they'll want to see the communication. Which the company will be compelled to provide. The device isn't relevant. (In my, admittedly limited, experience, this is how these processes actually work)

The original discovery request may be something along the lines of "please submit all communications that occurred in an electronic format via printed copies, blah blah leaglese". You then wait until the last day allowed in the discovery request, and then send over what ever print outs you have decided to release. If opposing counsel feels you've left something out (intentionally or otherwise), they can/will go to a judge to get a warrant. That warrant will not state "collect print outs of communications from personal devices". It will state "collect any personal electronic devices".

> If opposing counsel feels you've left something out

No, this isn't correct. If opposing counsel has evidence[0] that the personal devices have relevant communications not already provided, they can file for a subpoena to have you turn over personal electronic devices. Your counsel can file a counter-motion, and you'll probably have a hearing.

No where will there be a warrant, unless you're in criminal proceedings.

Like I said already: as far as I know, litigation holds and discovery are fairly common, and what you're describing is not. To clarify, this is all assuming you use a work profile and a personal profile.

[0]: I don't necessarily mean evidence in the legal standard, but they need some basis for this. "A feeling" isn't enough.