Functions , variables and their importance

What helps me identify functions , dependent and independent variables?
It is extremely important function helps us describe the situations we face , the dependent variables will help us understand the resulting effect of this situation to be tapped by the independent variables.
Functions represent any situation, let us analyze, interpret and predict certain behavior .
examples :
A trader who knows that more investment and purchase, the greater your profit, or determine that passes itself ad spending certain amount and see if that reflected in the number of customers coming to my business.
Another example as shown in the picture is to calculate employees ' salaries according to the number of total employees of a company .
Example behavior of wages

I think it helps us also to calculate laws like the " SUPPLY " regulating the prices of the products.