"EMS" has two different meanings in an. One EMS is the Intelletec 30A power management system and another is the Progressive Industries power line quality monitor. Unfortunate that the same name is used for two different electrical products, but nobody asked us RVers.
If you mean the display panel that shows Service Type (30A, 50A etc) and appliance power status (AC on. Heater ON, etc), That is the Inetlletec EMS and its handy but not necessary. It's value is to help manage amp load when your 50A coach is plugged to a more limited 30A outlet. It's main purpose is to save you from popping circuit breakers on the shore power post. It looks like the one in this manual:
If You mean the Progressive Industries shore power monitor, it's a safety device that protects your coach from poor quality or incorrectly wired shore power sources. It's excellent insurance against damage to electrical components. You may never encounter a shore power problem, but if you do, this kind of EMS will save you aggravation and money. A similar product is called the Surge Guard, avoiding the name conflict. It's easy enough to add one of these if you want one, either a portable unit or a permanent mount model.