7Zip is just a software to create compressed archives (like WinZip, WinRar, and even built-in Windows support for ZIP files). The files are compressable up to certain extent, depending on the file content. Media files (.MP3, movies etc) are already compressed, and there is little reason to waste time to re-compressing them again for 1-5% benefit.
A benefit of archives is that they can contain multiple files / folders stored inside a single file, which is handy for some projects like backing up of work items (text documents, spreadshets etc).
On the other hand - transfer rate to/from storage drive depends a little from what is being stored. It is the amount of the information which matters.
7Zip might have options which specify how hard to try to compress a file. Try for yourself which options work best for your files, and stick to them.