Julia has a lot of main competitors. I would consider Julia a competitor to Fortran, C, Matlab, R, and python. If you look at DifferentialEquations.jl or the clima.jl package, these are packages that are competing with low level libraries that would traditionally be written in C or Fortran. It competes with these by offering comparable performance, while having much better quality of life features (like automatic PGO, a package system, metaprogramming, and not having to deal with make files). It competes with matlab by having an incredibly rich linear algebra library, while being free and not making you do dumb stuff like 1 function per file.