You asked why we need

PYTHONPATH

line. Without it, the Jupyter web app will launch but it won't be able to launch a working ipython kernel:

[I 21:12:13.938 NotebookApp] KernelRestarter: restarting kernel (1/5), new random ports
/Users/myaccount/.pyenv/versions/2.7.17/bin/python2.7: No module named ipykernel