If aggregate demand and supply do not have a specific value, what is the point of the formula? I have been learning about aggregate supply and demand. I have just read this answer which states:

"Aggregate supply is a relationship of price level and output. It is a function, or a curve, or a table. It is not a single value. If we know a particular price level, then we can determine the level of output that would correspond with that. The GDP for 2006 is determined by plugging in the price level of 2006 to the AS curve for 2006, and seeing what output is produced at that price level."

I don't understand, I have learnt the formula AD = C + I + G + (X - M). Wouldn't that give a specific numerical value?
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.