So you don't have to carry two devices with you. Especially relevant for people who need to be contactable outside regular hours.

Also means work pays for the device, gives you a new one every year, and pays for and organises the data plan including worldwide roaming.

If you break it, they give you a new one in 5 minutes rather than haggling with an insurance company taking weeks...

It's a pretty sweet deal if you trust your employer.