Question

What does it mean and why (if at all) is it important?

Answer 1

It means you can add additional “operators” to a query. It’s important because you can do it extremely efficiently.

For example, let’s say you have a method that returns a list (enumerable) of employees:

var employees = GetEmployees();

and another method that uses that one to return all managers:

IEnumerable<Employee> GetManagers()
{
    return GetEmployees().Where(e => e.IsManager);
}

You can call that function to get managers that are approaching retirement and send them an email like this:

foreach (var manager in GetManagers().Where(m => m.Age >= 65) )
{
   SendPreRetirementMessage(manager);
}

Pop quiz: How many times will that iterate over your employees list? The answer is exactly once; the entire operation is still just O(n)!

Also, I don’t need to have separate methods for this. I could compose a query with these steps all in one place:

var retiringManagers = GetEmployees();
retiringManagers = retiringManagers.Where(e => e.IsManager);
retiringManagers = retiringManagers.Where(m => m.Age >= 65);
foreach (var manager in retiringMangers)
{
    SendPreRetirementMessage();
}

One cool thing about this is that I can change is at run time, such that I can include or not include one part of the composition inside an if block, such that the decision to use a specific filter is made at run time, and everything still comes out nice and pretty.