Of course it's about modularity, but how do you get modularity? How do you safely abstract the internals of libraries? Types and lack of side effects!
It's also about correctness. Most of the time spent writing any complex bit of software is generally spent finding out what's wrong with it.
Even philosophically this position is weak. It's self-evident to me you can make something more powerful/better by removing things. You can make something better by removing risks. You can make an UI saner by removing the possibility of invalid interactions. You can make art better by removing superfluous details.
I still think that types and control of side effects is essential to (good) FP.
- You can make limbs more useful by removing degrees of freedom, simplifying the control task.[2]
That's what I used to think, and in fact I still think like that somewhat, but do note it's explicitly mentioned as a red herring in John Hughes' well-known and often quoted paper "Why Functional Programming Matters".
From the paper:
> Such a catalogue of “advantages” [things that FP lacks or constraints] is all very well, but one must not be surprised if outsiders don’t take it too seriously. It says a lot about what functional programming isn’t (it has no assignment, no side effects, no flow of control) but not much about what it is. The functional programmer sounds rather like a medieval monk, denying himself the pleasures of life in the hope that it will make him virtuous. To those more interested in material benefits, these “advantages” are totally unconvincing.
> [...]
> Functional programmers argue that there are great material benefits — that a functional programmer is an order of magnitude more productive than his or her conventional counterpart, because functional programs are an order of magnitude shorter. Yet why should this be? The only faintly plausible reason one can suggest on the basis of these “advantages” is that conventional programs consist of 90% assignment statements, and in functional programs these can be omitted! This is plainly ridiculous. If omitting assignment statements brought such enormous benefits then Fortran programmers would have been doing it for twenty years. It is a logical impossibility to make a language more powerful by omitting features, no matter how bad they may be.
He then goes on to argue that the real benefit of FP languages is that they excel at modularity, a widely agreed upon trait of good software, and that they have an excellent mechanism for "gluing" stuff together, i.e. lazy evaluation.
(I've since read that some FP devs disagree about the importance Hughes placed on laziness).