[Tutor] environment variables

[linda.s]

Hi there,
I found the following paragraph in the web:
environment variables on Windows come in two flavors: user variables
and system variables. In particular, if there is a system variable
PYTHONPATH and you are adding this as a user variable, start with the
value in the system variable and add to it to define the user
variable, since user variables override system variables.

my question is:
since user variables override system variables, why we need do
anything related to python in the system variables? Why not delete the
value in the system variable and add to it to define the user
variable?
Linda