So the difference is that the thumbnail view helper accepts AssetInterface instead of ImageInterface, meaning it renders thumbnails for any kind of asset, not just images. This means it will also render a thumbnail for PDF or Audio files e.g., and here it will fallback to an icon. Back before we had thumbnail generators, you couldn’t control what it would render and thus the distinction was necessary as you might not want an icon rendered for non image assets.
However I do agree it’s a bit confusing now and it’s mostly a backwards compatibility thing, but the difference between the two is still there.
Even more confusion is that there’s now an additional thumbnail view helper that accepts AssetProxy’s needed for the new media browser.