The room sensors track presence and temperature by room to approach home heating and cooling more intelligently. However, it takes about 30 minutes for the sensors to update presence (motion). Essentially, they add temperature and motion sensors to other rooms and averages the temperatures for the overall temperature in the house.
Now the cool part is, say you are in the living room and all no one else is in any other room. Ecobee has a feature called follow me which basically, according to your settings, will only monitor the temperature in the room(s) that are occupied.
Another feature is, if the Ecobee has a set schedule and you have auto away/home enabled, it will over-ride the schedule based on actually presence that the sensors are reporting.
All of this can be done outside of the Ecobee hardware with various SmartApps available here on the community and you can even cook up your own recipes with webCoRE.
So you could essentially use the Ecobee therm only and add your own remote sensors which will work much better with motion given you instant updates on which rooms are occupied or empty.
However, if you don’t want to fool with all of that, adding the remote sensors allows the Ecobee software to do all of the work for you and give you access to all of your room temperatures in it’s own applet.