There are two reasons for a quant to know accounting. The first is that a fair number of "quant" problems are, or at least can be cast as, accounting problems. This includes time series or "non-parametric" analysis of accounting numbers.

The second reason is that accounting numbers are sometimes faked, meaning that a quant has to be able to deconstruct accounting numbers in order not to be fooled. Enron Corp. looked like a good investment to many quants; until you realize that their numbers were phony.