Why learn Java ? The answer to that question depends on your motivation for learning a (new) programming language.

Learning for intellectual gain

Compared to C/C++, Delphi, Ada, COBOL, Fortran etc, Java is a modern language. As a result it has been designed taking account of lessons learnt from limitations in those languages, and is very-much geared toward the academically acclaimed 'object-oriented' programming paradigm. It is a very popular choice of language in colleges and universities.

Learning for financial gain

In this case your decision may well depend on your viewpoint on the philosophies that brought Java into existence, such as platform independance and Microsoft anti--monopilisation. From a financial perspective, there are lots of well-paid jobs developing and maintaining Java (particularly J2EE) systems.