Why do some Java library methods delegate to a native method with a nearly-identical signature?

The native version is just an implementation detail.

This pattern separates the public interface of the method from the actual implementation.

I see at least 5 reasons why this is useful:

• test purpose (you can mock the java method call)
• alternative implementation: a particular version of the java library may implement that method in pure java form without call to the native implementation (common in swing code).
• backward compatibility: if in a new version, the method accepts some extra parameters, the original java method signature can be kept and the native implementation adapted.
• input/output validation: most of the time, before calling the native version, the java code will do some checks on the input parameters and throw exception if needed.
• isolation: the number of methods that directly uses the native code is limited, allowing more changes in the internal code structure.

You can probably find more advantages.