Using pip install -e ./bench-repo installs the bench from the locally cloned bench-repo directory. That is called the editable mode. Because you can edit the bench source code and it’ll reflect when bench command is executed. It is recommended if you’re developing bench itself or sending PR to bench repo.
If pip install frappe-bench is used, it’ll pull the pypi package and install it. You’ll not be able to do bench development. Recommended when you know editing bench is not required.