My parents wanted the ability to call me at any time - they really got annoyed that I wasn't going around with a mobile while downtown or going to work by bus.
Some workplaces want to know when you are being held up in traffic and won't be able to get into the office for an early meeting. Being able to call a taxi or emergency services is a big help, especially when there are no longer telephone boxes. Other employers are using Skype to handle IRC style discussions that don't require reserving meeting rooms.
Having a smartphone helps you navigate your way round a new town with unfamilar streets. The Google maps have helped me avoid getting lost several times. The worst time before I had the smartphone was when I went for dinner into a Burger King with entrances on two corners of the building, and ended up having my return path rotated 90 degrees, ended up in a completely different area of the city.