For the same reasons you need any programming language—to write useful software in!
> Currently I am learning Java…and Ruby
These are both good ideas. Java is widely used, reliable, and well tested for developing large applications. Ruby is simply a lot of fun to use, and one of the things you’ll discover while using Ruby is that it places a lot of functional programming idioms in an object-oriented context.
The issue with many popular languages is that they fall into much the same paradigm, namely imperative and either procedural or object-oriented, with some functional features borrowed for working with lists. It’s worthwhile to see that not all languages are like this, and learning a new language is not just about new syntax and APIs.
> but what about Haskell? I am genuinely curious what Haskell can offer.
Haskell will teach you to think about structuring your program in terms of simple data and transformations on that data. Object-oriented programming is about coupling data with behaviour, whereas a big part of learning Haskell is learning how to recognise high-level patterns in data structures and functions separately. In doing so, you can avoid the work of reimplementing common operations yourself.
You begin to say, oh, this is a data structure I can map a function over; I’d better make it an instance of Functor. And hey, I could use this data structure to represent this kind of computation, maybe it should be an Applicative and a Monad so I get some nice syntax for dealing with it. This function has a type signature that looks an awful lot like that other type signature—I wonder if I could implement one in terms of the other? And so on.
The type system and succinct syntax also make it very easy to create new data types, and you learn how this can help avoid bugs—don’t use a Boolean here, use an enumeration with two values; then you can’t mess up. Java discourages this by requiring more syntactic effort to create a new datatype.
A useful and oft-cited example of this is in the context of web frameworks: if you have separate data types for strings and HTML, that one simple thing eliminates a whole class of injection attacks. If you represent your links in the type system, then you can statically ensure that your site does not have any broken links. Simply put, quite a lot of bad things can be caught at compile time if you have a type system expressive enough to talk about them.
> I have yet to come across any notable project written with it.
There are more notable projects out there, but here’s my self-promotion. At my old job I maintained a compiler for ActionScript 3 in Haskell, as well as a build system that coordinated the compiler and other build tools to convert Flash games to iOS and Android applications.
Haskell is excellent at manipulating data structures—if that sounds ridiculously general, that’s because it is. The compiler is very nearly feature complete and, even with a certain amount of technical debt, was an order of magnitude smaller and easier to refactor than it would have been in an imperative language.
So there you go. From a recovering professional Haskeller, why you should learn Haskell. And, for that matter, the answer to “should I learn this language?” is always “yes”; but some languages should be higher on your list than others. ;)