Since they’re scanned from original, early prints, and then have any remaining damage hand-repaired and color correction applied based on non-faded contemporary sources, they’re probably the closest a person can get to seeing the originals as-projected today. Just projecting an original print wouldn’t get you nearly as close, since they’re all damaged and badly faded. Later official releases usually have both material alterations and weird color choices applied (often they’re just
really dark).
The 4Ks also include tons of audio mixes, including early shown-in-some-theaters ones like the mono mix, which sometimes differ substantially from one another (like using different takes from ADR)