You are putting it wrongly. You're implying that a backup/restore is a copy and an attach/detach is not. That's not correct.

I can:

- detach a database, copy the mdf/ldf/ndf files to a new instance/folder/drive/company, attach the files to a new instance with a new name. I can attach the original files to the original instance/db name.

- I can backup a database, copy that file elsewhere, have the first instance fail/retired, restore on a new instance. Move.

The difference is that an attach/detach removes the data/log files and database from the instance, or adds the data/log files and database to an instance. It requires the full size of the files, and all files to exist. It requires one copy of the files to move/copy the data to a new instance.

A backup allows the original to continue to be used while a copy of the data and objects, and only a copy of those objects, is made. This is usually less space than the database file sizes. If you copy this to another instance, to get it to a database (new/refresh), you must do another copy in which the data is copies into the mdf/ldf/ndf files.

Stating anything else about moving, copying or "why" you do something, is incorrect or incomplete. It's also unnecessary. The differences are technically how things are handled, not the purpose or necessarily the manner in which the technique is used.