There is one good answer for what a general purpose language like Haskell is good for: writing programs in general.
For what it is used for in practice, I've three approaches to establishing that:
- A tag cloud of Haskell library and app areas, weighted by frequency on Hackage.
Indicates that it is good for graphics, networking, systems programming, data structures, databases, development, text processing ...
- Areas it is used in industry - a lot of DSLs, web apps, compiler design, networking, analysis, systems programming , ...
And finally, my opinion on what it is really strong at:
- Problems where correctness matters, domain specific languages, and parallel and concurrent programming
I hope that gives you a sense on how broad your question is, if it is to be answered with any specificity.