Because at least a second screen adds some flexibility and means people might have the best of both worlds,a device small enough to fit in a pocket,but if they want a bigger screen to do something else,it avoids having to lug around a tablet. Now its not perfect due to the bezel between them,but at least its a start and I can even understand a phone costing more due to that. Apparently Samsung is working on a phone with a truly flexible screen,so it will be interested to see what can be done with that.
Look in all the sci -fi stuff and people using personal communicators with pop up screens which are not the size of tablets,etc.
With cameras on phones they seem to overhype every little "improvement". I mean one company shrunk the sensor down to save on costs,then on purpose buried the fact,and then harped on how they used glass lens elements,when webcams have had glass lenses elements. Once in a while you do get an actual effort on the part of a company,but it tends to somewhat get buried over the avalanche of other "revolutionary" camera improvements.