For me, it's not the need to feel "in contact" 24/7 as much as it is to enable people to call my number where ever my work has happened to assign me for the moment.
Currently I am in the DC area. But likely as not I will be either down south or in the mid-west for the next week or two.
When I need to connect to the net from my computer and I do not have normal access, I simply turn my phone into a hot spot and connect. Should the fiber to the house go out, I can hook my phone up to my router and all devices on the network now have access to the outside world.
Even if I didn't need a cell phone for work purposes, I'd still have one over a land line as it gives me far more flexibility.