Not a lot of it is statistical arbitrage - yes this is what Rennaisance and D.E. Shaw do, but this is not what banks (sans perhaps Goldman Sachs) devote much of

Not a lot of it is statistical arbitrage - yes this is what Rennaisance and D.E. Shaw do, but this is not what banks (sans perhaps Goldman Sachs) devote much of their compute power to. It's just too hard to make money doing this compared to more traditional activities like market making. As stated, risk management is a big application: VaR and market stress scenarios are computationally intensive, particularly for portfolios with path-dependent derivatives. Pricing is the other big application: it is similarly computationally intensive to value derivatives against the market-implied term structure of volatility.