Backdooring Popular Software
Suppose further that you don't have the source code.
Now suppose that a backdoor / virus / malware is pushed out on a given day to all these devices with the intention of bricking them.
Suddenly the internet is down (servers), the power grid is down, the phone network is down. Everything that runs this software is dead. Within 24 hours almost all computing-based things stop.
It can take weeks to disassemble, reverse engineer, find malware, and develop a fix. Doing so without communication (internet, phone) would require someone with a great deal of skill on an isolated, generator based, non-networked machine with all of the required software to support the task. Hopefully the bricked machines can be rebooted and recovered but hand-distributing the fix will take many months.
Or suppose you want to know the position and motion of every asset in government and business. Every mobile phone begins reporting every location and every motion. What possible use would it be if war broke out to know the position and strength of every asset?
Or suppose you wanted a distributed denial of service attack from every device running this software.
Or even such benign tasks as scraping every phonebook on every phone to construct a call graph of who-calls-who.
That can't happen of course. It's kittens all the way down. What could go wrong?