Is valvuloplasty performed to allow an adult patient over 80 to have more strength for valve surgery?

Yes. This is done on occasion in some centers, the patient that does well immediately post procedure, usually can survive a few months and perhaps get more fit for the more extensive operation of aortic valve replacement. Most of the time however, the valve is replaced without an attempt at repair, as the repair itself carries substantial risk (stroke, death, need for emergent surgery).
Yes. In certain situations when the heart muscule is weak or the patient is too frail to withstand open heart surgery for severe aortic stenosis, valvuloplasty is preformed as a bridge to improve the heart function and overall patient condition prior to open heart surgery. Some centers are replacing the valve entirely through the groin for high risk patients. This is called tavr.