Tempering valves, also known as thermostatic mixing valves (TMV's) blend hot and cold water to deliver tepid water to plumbed-in eye washes, drench hoses, and safety showers. Even in indoor applications, standard cold water supply lines may deliver water so cold during the winter months that the user can not withstand being under a safety shower for the recommended length of time (up to 15 minutes), causing them to stop first aid treatment prematurely. For this reason ANSI Z358.1-2014 (Section 6.4.6; B6) states that water delivered by emergency equipment be "tepid" which is defined as between 60 °F (16 °C) and 100 °F (38 °C) except when this would accelerate an adverse chemical reaction.
As Z358.1 is incorporated by reference into the International Plumbing Code (IPC) and this means that tempering valves are basically required under most state and local plumbing codes. But even if they aren't required, based on our firsthand experience in actual emergency situations, we consider them be a Really Good Idea and best practice.