For me, functional programming permits better means of reasoning about the flow of data through software systems. You always know how variables came to have the

For me, functional programming permits better means of reasoning about the flow of data through software systems. You always know how variables came to have the values they do, because they were passed into the function that way. This reasoning extends back up the call stack. It makes debugging much easier and therefore cheaper than with OOP for example. I think a functional approach is just more predictable generally.