The Iron Fly is a defined risk, neutral bias trade that shares properties of both an iron condor and a butterfly, hence the name, Iron Fly. The combination of a short call spread and a short put spread is similar in nature to an iron condor, whereas the fact that the short strike on both the call spread and the put spread is the same is very much like a butterfly.
The best case scenario for this position is for the underlying to stay right at this shared short strike, but the strategy can still be profitable as long as it stays above the long strike of your put option and below the long strike of your call option. In fact, the profit and loss diagram for an Iron Fly is identical to that of a butterfly. The decision to choose one strategy over the other can then really be boiled down to your desired risk/return dynamic and probability of profit, as the two strategies differ in these regards. Much like any strategy where we sell premium, we prefer to put on an Iron Fly in a high implied volatility environment.