Many home automation task have to be done depending on whether somebody is at home or not. Heating the bathroom up in the morning is a good thing while somebody is at home but should be avoided while we are on a long vacation.
Furthermore, if openHAB knowns that somebody leaves home, it can take care of energy saving actions such as reducing the temperature or switching all lights off.
Other scenarios are imaginable but these two examples are the use cases I have currently implemented in my openHAB rules.
Since we both (my girlfriend and I) have our iPhones with us almost all the time, it should be quite easy to track our presence using these devices.