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 opened file, it just removes them from the Collection clicked on. If an object is in more than one collection it will still remain in others, otherwise it may become orphaned data.
Orphan data is "garbage collected" and may be removed on saving file.
Put Outliner in Orphan Data Mode to check before saving to avoid losing important data.
Deleting fully erases the object and its geometry, while also removing it from all the collections it belongs to, from the scene and from the file, reducing file size and memory consumption.