I don't know what I would need 3 or 4 phones for, I don't make/take enough calls for just 1 anymore lol
Well it's always good to have at least a backup phone, all set up and ready to go in case something happened to the primary.
Those who have more than one backup is probably for emotional/hobby purposes. In most cases, a 3-4 year old phone is worth more keeping around than selling for practically nothing.
Or sometimes you want to have one you can use while your primary is charging up.
Or, you want different devices for completely different purposes. For example, I have my SIM card in my BlackBerry KEY2 which serves as a primary communicator thanks to the keyboard and notification LED, and my S10e is there for anything that involves actually enjoying content, because a 5.8" Samsung AMOLED is FAR greater than a 4.5" TCL LCD.
I came to the conclusion that the only way to have both worlds is to have two separate devices...at least until someone makes a decent slider.