Because it enables an equivalence between functions and objects. Not a technical one that's hidden deep in the implementation, but a practical one that's apparent to library authors and users.
If you want to make any object `foo` callable with first-class application syntax, i.e. `foo(bar)`, you add an `apply` method to that `foo`. You don't need to bother extending Function, you don't need to come up with a method name and call that. When you call `foo()`, you don't know if `foo` is a function or an object, and you don't care. You can find out easily with a half decent editor of course, but the point is, you're free to mix objects and functions as you wish, the language does not get in your way.
And for all that it provides, it's a complete non-issue in terms of learning curve. It's a basic language feature that you learn once and are never impeded by thereafter.
