If you're comfortable using R and the tooling and performance it offers, you should probably keep using it. Julia isn't just about big data, but it does tend to appeal to people who are struggling with problems that are sufficiently hard that their old tools left them in a world of pain. That pain may comes from size, complexity, CPU-intensiveness, or need for more language expressiveness (a hard thing to define). In particular, Julia offers a unique combination of productivity and speed for numerical work that can't be found anywhere else.