Although the Imperative programming paradigm is the most popular among [professional] programmers, it is just one of several means of "giving orders" to a computer. In addition to it and Functional , we also have Logical Programming (eg Prolog), Dataflow .: the internal engine of a worksheet), Function-Level (programs do not manipulate data, but other programs), etc. And, of course, the most conceptual / theoretical Turing Machine - to which all previous paradigms are computationally equivalent.
Each of these paradigms has advantages and disadvantages, and it is common as programming languages "evolve" that they incorporate aspects of other paradigms. The main reason for using one paradigm or another is its expressivity : at first, we could all be programming in Turing Machines, but beyond the difficulty of reading / writing / understanding such programs they would also be too extensive in relation to what they make useful. Similarly, different paradigms offer a greater degree of expressivity for specific domains (although the language as a whole is Turing-complete).
As for F # in particular, I'll quote an answer to a similar question in the Stack Overflow in English:
My expectation is that F # will be used for parts / pieces of some specialized systems - parts that involve complex threading / math / financial calculations / modeling / etc, where F # fits well. In most other areas (UI, Data Access Layer, etc.), a general-purpose language like C # seems (in my opinion) preferable.
One of the advantages of F # is that (in theory) you can prove that the code is working, instead of just testing it. Threading support (thanks to immutability and asynchronous use of
! ) is also good (although PLINQ can compete in threading ).
(Comment: I disagree with the author of this answer in the sense that it is possible to prove that an imperative program is correct - it is only more laborious ...)
I can not comment on our market, since I do not participate in the Microsoft "ecosystem" (preferring to develop in free software), but in general opening the minds of Brazilian programmers to functional programming I believe would be very well coming!