Now you might be thinking that if a thief ever wants to sell your stolen phone, they'll have to turn it back on at some point. This is true.
But if they turn it on in a remote location away from other in-network phones, and they promptly enable airplane mode, offline finding still won't work. They could also turn it on in a Faraday cage while they use other tools to wipe or unlock the device. Or they could simply boot it into recovery mode, DFU mode, or Fastboot to run wiping utilities before the phone's operating system even starts.
So turning off a stolen phone is still the best way to avoid getting caught. It's such a simple measure that any thief can take to make your phone disappear forever.