Haskell is useful for just about everything that Python, Ruby, Java, C#, etc are useful for. For the last 5 years my day job has been writing Haskell. Most of that has been web apps, but there have been other things like machine learning applications, a nice command-line interface to Amazon Redshift, an automated ETL tool for a large production application, and most recently a complex interactive browser app (using GHCJS to compile Haskell to Javascript).