Sometimes, original pressings sucked to begin with and the new ones were given attention by people who love them (like Led Zep). In the Court of the Crimson King was a bad pressing when I got it in the 80s, and there were several generations of gawd awful CDs. Virgin had lost the masters and consumers got third and fourth generation tape copies (see the King Crimson site for the whole story). They eventually found them and Robert Fripp recovered the original master tapes a couple years ago, and the 200g reissues are wonderful.
There's plenty of information out there. If you're inclined to buy, check the critics but especially the customer reviews. If it's an album you love, it's usually--not always but usually--worth it. Kinda/sorta/on the fence about it? Buy a used copy.