When and why should we use the urdf transmission tag

[fjp]

When and why should we use the urdf/xacro transmission tag when working with ros_control?

According to the C++ API documentation:

The transmission_interface is not used by controllers themselves (it does not implement a HardwareInterface) but instead operates before or after the controllers update, in the read() and write() methods (or equivalents) of the robot abstraction.

Can the transmision tag be omitted or is it always required?

According to this answer and the ROScon 2014 talk:

Actuators are connected to joints via mechanical transmissions, and you need to use them if you want ros_control to handle the mapping between joint and actuator spaces.

Is the transmission always required when working with the gazebo_ros_control plugin?