Why should I want to learn haskell?
(Surely this is a duplicate question, but I can't find one now.)
You learn it in order to learn pure functional programming, which forces you to do many things in a completely different way. You get a new way of thinking. Programming without state? Programming without effects? Everything is lazy? Crazy type system with type inference? What the hell are monads? Your mind will be repeatedly blown, but in the end you come out with new perspectives/techniques from functional programming that are hard to otherwise pick up without going full-blown Haskell.
The problem with trying to be specific, is that trying to tell a non-Haskeller what they'll learn from Haskell is like trying to explain the color "green" to a blind guy.
A few years ago, many people were surprised to discover one of the introductory courses was being taught using Haskell as a/the programming language! Although I didn't have experience with Haskell, I had some background in Lisp and other Functional Programming languages.
I think the anecdote shows how knowledge can be useful when you least expect it.
In more practical terms: You may have noticed that CPU speeds hit a wall some years ago, and now the most practical way to pull more performance from computers is by installing multiple CPUs. Now it so happens that most if not all of the programming languages you know are essentially single tasking, and subject to the Von Neumann bottleneck. An obvious solution is parallel programming, but that can be very painful if the parallel parts of your program end up sharing state, i.e. memory - and this is most often the case.
It turns out that Functional Programming is a style that allows you to mostly circumvent the problems of parallel programming with shared state. Stated differently, it's fairly easy to write programs in the FP style that are "naturally" thread safe and suitable for parallel processing. Depending on the language, compiler and hardware you may even find (as I recently did) parts of your program running in parallel without ever having done any explicit coding for parallelism.
I'm frequently wrong, but my guess is that Functional Programming will turn out to be one of the hot programming paradigms of the future as parallel programming becomes more important and more difficult. Haskell may not turn out to be the language of choice - my personal favorite is currently Clojure - but it may well be worthwhile to take a look at one or more FP languages.
I also don't think that I'll be easily able to plug a piece of Haskell code into game engine.
If you only want to write 3D game engines then maybe there's not much point in learning Haskell.
If you want to be a well-rounded programmer capable of programming in multiple paradigms and you currently only know C-like languages then it is worth a look.
Every time you learn a new very different language it makes learning the next language easier because you're not just memorizing new syntax, you're also learning different ways of thinking about programming. If you try out a new language and you see some new feature you will more quickly understand it if you can relate it to another feature in a language you already know. The more languages you know the more likely it is that this new feature is similar to something you've seen before.
It's also handy to have many tools available in your toolbox. Some problems are better solved in one language than another. If you have 5 very different types of languages then you can select the best one for each problem. If you know only 2 or 3 very similar languages then some problems will be easy to solve, but others might be more difficult than if you used a language which is better at that specific task.
If 3D programming is your thing, you might be interested in some slides from a talk entitled The Next Mainstream Programming Languages: A Game Developer's Perspective by Tim Sweeney, the founder of Epic and technical director for the Unreal engine. He's spoken on the subject multiple times, and he clearly thinks very highly of Haskell.