Bit of a newbie question; but I see this pop up from time to time.
If we have a volatility surface (e.g. for the S&P500) built from market options what more can we do with it, but price other European options on non-traded strikes and maturities.
More specifically, I see people claim to need the volatility surface to value exotic options and risk management. How do they do this?
Note, as I understand it, if we have a Heston Model (for instance) calibrated to options prices, we can value any exotic option we'd like and compute some gradients to our liking. But, we can't get there from only picking out an implied volatility of the European options.
As an example question: Given the vol surface - how do I price a barrier option on the SPX? How do I compute its sensitivities to risk-factors such as spot, vol, etc..
What am I missing here?