What is an instance collection ?

It's an object which refers to a collection content linked from another file, or from your main file.
This way, you can get multiple copies of linked data very easily.

What is this toolbox for ?

This toolbox contains some helpful tools when you are using instance collection, but it also works for meshes :

• renaming from its data : the object or instance collection will be renamed regarding to its data (mesh or collection)
• offset collection center from selection : the "center" of each selected objects' collection will be well set
• convert meshes to instance collection : each selected object will be converted in an empty instance collection, which allows you to use collection instead of meshes
• randomize elements : this tool will change randomly from your selection to element in a defined library
• convert selection into a new instance collection
• and many more to come ! (one day)

Why it's usefull ?

Because Instance collections is one of the best feature of blender.