if the required message is a standard message type available with ROS, do we still need to define it in msg/xxxx.msg?

edit

You can skip it. All standard message types are already run through gencpp/genpy/gennodejs/etc. during build time of the ROS distribution packages to generate the c++, python, and node.js message definition classes. You can verify this using locate on your machine:

> locate WrenchStamped
/opt/ros/melodic/include/geometry_msgs/WrenchStamped.h
/opt/ros/melodic/lib/python2.7/dist-packages/geometry_msgs/msg/_WrenchStamped.py
/opt/ros/melodic/lib/python2.7/dist-packages/geometry_msgs/msg/_WrenchStamped.pyc
/opt/ros/melodic/share/common-lisp/ros/geometry_msgs/msg/WrenchStamped.lisp
/opt/ros/melodic/share/common-lisp/ros/geometry_msgs/msg/_package_WrenchStamped.lisp
/opt/ros/melodic/share/gennodejs/ros/geometry_msgs/msg/WrenchStamped.js
/opt/ros/melodic/share/geometry_msgs/msg/WrenchStamped.msg
/opt/ros/melodic/share/roseus/ros/geometry_msgs/msg/WrenchStamped.l
/opt/ros/melodic/share/rviz/icons/classes/WrenchStamped.png

A standard message type does not need to be defined in the msg folder. Only custom messages need to be defined there.