The app evaluated the expected income from Ancillary Service contracts in the UK energy market and subequently reconciled against actual income:
https://en.wikipedia.org/wiki/Ancillary_services_(electric_p...
It comprised a windows server, a web app and a calculation engine.
As well as remorselessly removing boilerplate code using FP techniques the F# solution also allowed us to explore our way, via the REPL, to solving the problems we were faced with and come up with better ways of doing things. The clarity, concision and low ceremony of F# code allowed us to rapidly evaluate and change approaches if necessary.
I find getting into the flow of development much easier in a language that supports this exploratory approach to programming - leads to less rigid thinking.