It is very easy to miss noticing an idle icon above a goods building, especially in the later age cities which are larger and contain many more buildings. The presence of an idle icon in the side bar draws your attention to the face that one (or more) buildings remain idle. By clicking on the idle icon, it would cycle through the buildings not currently in current production - be they supplies, goods or military buildings. If the player intended for the goods building to remain idle and not start a production cycle for whatever reason, you would just skip past that when clicking on the icon to view the next idle building. In the same way that you may not always want to retrain troops - or even set supplies production.
Not having the idle icon for goods buildings often resulting in a building standing idle when it should not, and valuable goods productions being lost.
For more information and to view the overwhelming support for this, please see the following link:
http://forum.en.forgeofempires.com/...in-the-side-buff-bar-for-idle-goods-buildings