What is the "nomkl" Python package used for?

The nomkl package is not a Python package, but instead what the Conda documentation refers to as a mutex metapackage. Its sole purpose is to toggle the use of different build variant configurations in an environment. In this specific case, it controls whether other packages (such as NumPy) use Intel's MKL library for their BLAS implementation or something else (e.g., OpenBLAS). That is, many other packages link against the BLAS library during build and so there are equivalent versions of packages like NumPy that reference MKL or OpenBLAS symbols. The Anaconda documentation on MKL Optimizations that @deceze dropped in the comments has details on nomkl specifically.