When you call boo function in Haskell, you have the power to create the "universe" which is both necessary and sufficient for the function to perform some self-contained computation. When enough of these computations are composed and nested together, you can really see how data flows through some logical procedure, possibly one with lots of essential complexity. On the other hand, when I look at the same signature translated into Java, for all I know it could be the 4th function call in a stateful 8-function-long set of functions you always have to call in order, except you don't have to call the 6th function if you pass true to the 2nd function. Easy enough, right?
The fewer combinations of arbitrary state are allowed to determine the behavior of your program, the fewer unanticipated scenarios arise. I think this is a good software engineering principle. The corollary that I think Haskell really brings home when it comes to designing programs is that it's better to change the way data flows in one small section than to add a little more state somewhere and handwave away state combinations which you think are impossible. This idea applies in both Haskell and Java.