I have tried to use 'unlink' in outliner editor, because I didn't know what it is exactly. And then I thought that it did same thing as 'delete'. But there must be differences between them. What is it?
Unlink removes objects from collections, Delete erases them permanently.
Unlinking doesn't remove the object or its geometry from the file, it just removes them from the Collection you clicked on. If an object in in more than one collection it will still be kept on others, otherwise it will be unlinked from the scene and become orphaned data, but not removed from file.
Deleting on the other hand fully erases the object, from all the collections it belongs to, the scene and the file, reducing file size and memory consumption.