Do I need to manually call Resources.UnloadUnusedAssets?

This more of a general question but I really couldn't find an answer for it.

I have a game where I call Resources.load in many places. For example:

TextAsset textAsset = (TextAsset) Resources.Load("items.xml");

If Resources just returns an object then what is the need to do unloading? I mean when the function is finished the "textAsset" local variable is destroyed, isn't it?

So do i need to manually call Resources.UnloadUnusedAssets after each time I do Resources.load? If yes, will switching a scene do this automatically?