Most users have their WIFi on all the time and even have automatic connection to strongest signal turned on.
The issue being, connecting to WIFI, doesn't mean you are connected to the Internet.
Most WIFI Routers have a password required and even most public WIFI still requires you to Accept the Terms and Conditions to use it.
Even many modern office printers, emit a WIFI signal, generally so you can print direct to that printer.
So turning your WIFI off on your mobile device, ensures you are always connected to your Mobile Data, a internet connection you can guarantee is connected to the internet, as long as you have a Mobile Data activated and have credit on your account.
We've seen cleaners phones connecting to Printers in offices and simply turning off the WIFI, and staying connected via the Mobile Network instantly resolved all issues.