Perhaps, but it's quite convenient for any software that wants to ingest PSD files without the (nigh-intractable) challenge of reimplementing the entirety of Photoshop's parser and rendering systems!

The "preview" is actually the main image data from the version of the file format that was used before Photoshop supported layers.

It remains there, I guess, so that you can display the image even if you don't support loading and compositing layers.