This folder contains objects that have purely decorative purpose, so that they can be added and removed from any scenario without having any impact on the gameplay.
That means that if you decide to give objects in this folder a meaning beyond decoration, you should move them out into a proper place and also remove them from the proplist "Environment_Attributes" found in Ambience.ocd/Script.c - by the conventions you are guaranteed that nothing will break when you do that.
For example you could move Ambience_Zicadas into Animals.ocd, give them the ID "Zicadas" and remove the "Zicadas" entry from the proplist - or just set the "ID" field of the "Zicadas" entry to "nil".

You are encouraged to use objects from this folder through the function "CreateEnvironmentObjects" which guarantees full backwards and forwards compatibility. However, all objects in this folder also have Place()-methods if you need to place them manually.