BLAS thread count vs Julia thread count

On macmini with Julia 1.6, I am able to set more BLAS threads

julia> BLAS.get_num_threads()

julia> BLAS.set_num_threads(10)

julia> BLAS.get_num_threads()

Does it really work? Can you see the performance difference? Was this the dmg you got from Julia downloads or did you compile directly on your mini?