To address the original question, here are some comments.
1. First, remasters are a marketing tool by the record companies to sell more copies of mature material that has fallen off the radar. By having a well known mastering engineer put out a new version the record company gets to highlight the old record and many of us will by the same album again - maybe many times over. I have at least 6 versions of CSN's first album on vinyl and CD. I don't even know how many versions I have of DSOTM.
2. For vinyl, remastering can allow the final master (please don't use the word, "mixed" - that is a completely different step) to have deeper bass and greater dynamics that can take advantage of state of the art vinyl pressing equipment and electronics. Many vinyl reissues are pressed on 180 gram vinyl and the stampers are used on fewer copies for a higher quality transfer. Several companies are also making 45 rpm lps of classic albums which allow for significantly higher dynamic range. It also takes advantage of modern high quality tonearms and cartridges. 40 years ago a cheap cartridge and tonearm wouldn't track a record with lots of bass - it would literally jump out of the groove.
3. For CDs there are several steps in the process where you can get an improvement with a new master. The first one involves the A/D converters that turn the analog master tape into a digital file. If the original CD was made decades ago you can bet that modern converters sound better. The old version may have been converted at 16/44.1 but the new version can be digitized at 24/192 or even higher - offering the opportunity to sell the record as a high resolution download. Second, the new mastering process may use a better sounding tape deck and electronics than the original version. Third, the mastering engineer may alter the gain, EQ, compression and limiting using the final mixdown tape from the recording studio. He/she can also utilize processors to manipulate things like stereo processing (sound stage width, etc.) and soft clipping of extreme transients. The goal is that hopefully the new version will sound better than the old one.
Of course, the question is whether or not the new version is actually better. Unfortunately many remasters are more heavily compressed (by using a limiter) to make them sound "modern." You may like this or you may not. For me a heavily compressed, limited recording almost always sounds worse. Limiting will likely overwhelm all of the other positive factors of remastering and make the version worse than the original.
Because of the above, buying remastered versions of older popular music is a bit of a hobby in itself. I usually check on the Steve Hoffman forum to see if the remaster has been reviewed by the members and I check the Dynamic Range (DR) database to see how compressed it is. Many vintage recordings are available in a surprising number of versions and some of them are collector's items going for collector prices. You can see this on Discogs. I fully admit that I'm a version geek and for me it's a fun part of my audiophile obsession.
Sorry for the long answer but I did try to keep it under 10 pages.
1. First, remasters are a marketing tool by the record companies to sell more copies of mature material that has fallen off the radar. By having a well known mastering engineer put out a new version the record company gets to highlight the old record and many of us will by the same album again - maybe many times over. I have at least 6 versions of CSN's first album on vinyl and CD. I don't even know how many versions I have of DSOTM.
2. For vinyl, remastering can allow the final master (please don't use the word, "mixed" - that is a completely different step) to have deeper bass and greater dynamics that can take advantage of state of the art vinyl pressing equipment and electronics. Many vinyl reissues are pressed on 180 gram vinyl and the stampers are used on fewer copies for a higher quality transfer. Several companies are also making 45 rpm lps of classic albums which allow for significantly higher dynamic range. It also takes advantage of modern high quality tonearms and cartridges. 40 years ago a cheap cartridge and tonearm wouldn't track a record with lots of bass - it would literally jump out of the groove.
3. For CDs there are several steps in the process where you can get an improvement with a new master. The first one involves the A/D converters that turn the analog master tape into a digital file. If the original CD was made decades ago you can bet that modern converters sound better. The old version may have been converted at 16/44.1 but the new version can be digitized at 24/192 or even higher - offering the opportunity to sell the record as a high resolution download. Second, the new mastering process may use a better sounding tape deck and electronics than the original version. Third, the mastering engineer may alter the gain, EQ, compression and limiting using the final mixdown tape from the recording studio. He/she can also utilize processors to manipulate things like stereo processing (sound stage width, etc.) and soft clipping of extreme transients. The goal is that hopefully the new version will sound better than the old one.
Of course, the question is whether or not the new version is actually better. Unfortunately many remasters are more heavily compressed (by using a limiter) to make them sound "modern." You may like this or you may not. For me a heavily compressed, limited recording almost always sounds worse. Limiting will likely overwhelm all of the other positive factors of remastering and make the version worse than the original.
Because of the above, buying remastered versions of older popular music is a bit of a hobby in itself. I usually check on the Steve Hoffman forum to see if the remaster has been reviewed by the members and I check the Dynamic Range (DR) database to see how compressed it is. Many vintage recordings are available in a surprising number of versions and some of them are collector's items going for collector prices. You can see this on Discogs. I fully admit that I'm a version geek and for me it's a fun part of my audiophile obsession.
Sorry for the long answer but I did try to keep it under 10 pages.