If aggregate demand and supply do not have a specific value, what is the point of the formula?
This should have been a comment, but I have less that 50 reputation and I am new to this site as are you. 

First, I think the point is clear and concise, but you may be misinterpreting it slightly. 

Think of demand or supply like a pendulum swing that is being pulled by two forces. 

For supply this is prices and output, prices can be pulled by output and output can be pulled by prices. 

However, you're right, you can get specific values, you can use the formula. 

The formula is particularly useful for averages, the prices and output over time.