In my job as FX trader we use as option pricer a variant of B&S.
We use that model for “accounting” purpose, i.e. for storing the daily P&L of the portfolio, and also for control the trading limits in term of greeks.
This make sense to me because FX options are OTC instruments.
However, I’ve never understood why we need a pricer for options traded on the exchange, eg equity option: in that case we could simply store the P&L based on the market value of the option.
Can you explain to me what I’m missing?