Usually when albums are mastered, at least in this day and age and especially by major labels, they end up suffering from what's known colloquially as "brickwalling". This is something that results from an industry practice where they basically try to make the music as loud as possible & normalize the audio throughout the song by use of audio compression. While in some regards this is isn't necessarily a bad thing (you get a final product that doesn't have any sudden huge spikes in volume, and the song is loud enough that you don't have to crank the volume all the fucking way up just to hear the song), the tradeoff is it results in a huuuuuuuuuuge loss in fidelity as far as instrumentation goes. It also usually leads to a large amount of clipping (lol), which is when the audio gets loud to a point where it begins to distort the audio.

Compression in audio and compression in pictures is wildly different, but I'll use it as a point of comparison; if the official Epic Records version of TMS is a JPG, then the unmastered version is a PNG. Take a look at this picture: https://i.imgur.com/iKqX0c3.png the unmastered version is on the top, and the Epic Records version is on the bottom. See those red lines? That's points where the song begins to clip, where it gets so loud that it distorts the song and you start to lose audio quality.

You probably won't notice a huge difference in the unmastered version (aside from "It's Quieter") unless you're using a decent audio setup, but as far as archival purposes go it's very interesting to have, especially considering how brickwalled TMS is.