For example, you might have a rule that routes your incoming calls to your mobile phone whenever your availability status is "I am Away." However, today you plan to be in the office, but not at your computer. As a result, you have an "I am Away" availability status much of the day. If you select "Office phone" as your preferred device, then calls you get do not go to your mobile phone, but instead ring your office phone, even if you are away from your desk.
You might then go to another building. If you have a rule that routes your calls to your mobile phone if you are not at your office desk, when you take your computer to the new location, the new location is detected. Thus, your preferred device automatically updates to your mobile phone, according to the rule you created.
Parent topic: Troubleshooting calls