Closures: why are they so useful?

IMHO it comes down to being able to capture blocks of code and their context to be referenced at some point later on and executed when/if/as required.

They may not seem to be a big deal, and closures definitely aren't something you need to get things done on a daily basis - but they can make code code much simpler and cleaner to write/manage.

[edit - code sample based on the comment above]

Java:

List<Integer> numbers = ...;
List<Integer> positives = new LinkedList<Integer>();
for (Integer number : integers) {
    if (number >= 0) {
        positives.add(number);
    }
}

Scala (skipping some of the other niceties like type inference and wildcards so we're only comparing the effect of the closure):

val numbers:List[Int] = ...
val positives:List[Int] = numbers.filter(i:Int => i >= 0)

Here are some interesting articles:

• Crossing borders: Closures - by Bruce Tate, mentions some advantages of closures
• A Definition of Closures - by Neal Gafter (one of the people who invented one of the closures proposals for Java)