Why are there finalizers in java and c#?

Hmya, you are getting a picture painted here that's a bit too rosy. Finalizers are not guaranteed to run in .NET either. Typical mishaps are a finalizer that throws an exception or a time-out on the finalizer thread (2 seconds).

That was a problem when Microsoft decided to provide .NET hosting support in SQL Server. The kind of application where restarting the app to solve resource leaks isn't considered a viable workaround. .NET 2.0 acquired critical finalizers, enabled by deriving from the CriticalFinalizerObject class. The finalizer of such a class must adhere to the rulez of constrained execution regions (CERs), essentially a region of code where exceptions are suppressed. The kind of things you can do in a CER are very limited.

Back to your original question, finalizers are necessary to release operating system resources other than memory. The garbage collector manages memory very well but doesn't do anything to release pens, brushes, files, sockets, windows, pipes, etc. When an object uses such a resource, it must make sure to release the resource after it is done with it. Finalizers ensure that happens, even when the program forgot to do so. You almost never write a class with a finalizer yourself, operating resources are wrapped by classes in the framework.

The .NET framework also has a programming pattern to ensure such a resource is released early so the resource doesn't linger around until the finalizer runs. All classes that have finalizers also implement the IDisposable.Dispose() method, allowing your code to release a resource explicitly. This is often forgotten by a .NET programmer but that doesn't typically cause problems because the finalizer ensures it will eventually be done. Many .NET programmers have lost hours of sleep worrying whether or not all Dispose() calls are taken care of and massive numbers of threads have been started about it on forums. Java folks must be a happier lot.

Following

If you read the JavaDoc for finalize() it says it is "Called by the garbage collector on an object when garbage collection determines that there are no more references to the object. A subclass overrides the finalize method to dispose of system resources or to perform other cleanup."

http://java.sun.com/javase/6/docs/api/java/lang/Object.html#finalize

So that's the "why". I guess you can argue whether their implementation is effective.

The best use I've found for finalize() is to detect bugs with freeing pooled resources. Most leaked objects will get garbage collected eventually and you can generate debug information.

class MyResource {
    private Throwable allocatorStack;

    public MyResource() {
        allocatorStack = new RuntimeException("trace to allocator");
    }

    @Override
    protected void finalize() throws Throwable {
        try {
            System.out.println("Bug!");
            allocatorStack.printStackTrace();
        } finally {
            super.finalize();
        }
    }
 }

In .Net land, t is not guaranteed when they run. But they will run.