If your car gets stolen, and the police find it, they will call you from a phone number that's not in your contacts. If you don't pick up, you won't realize that your stolen car has been recovered a couple miles from your house, and if you show up there in 30 minutes you can drive it back home, but if you don't, the police will send it to a towing yard, which will require you to go through 24 hours of paperwork with the police to obtain a release and then pay the towing yard $1,000+ to tow and store your car.
If you live in an area of low crime, though, maybe it'll be fine not to answer phone calls from numbers that aren't in your phone.
If you live in an area of low crime, though, maybe it'll be fine not to answer phone calls from numbers that aren't in your phone.