As you said yourself, you have no performances problem with R and Python, so I do not think you need to learn Julia.

However, in fields dealing with larger datasets (e.g. genetics, astrophysics, ...), Python and R are real bottlenecks in data processing pipelines, and are thus often replaced by programs in C/C++/Java/etc. Having a language with the expressiveness and dynamism of Julia on the one hand, and performances in the same order of magnitude than C++ in the other hand is a huge plus.