From my experience there are 2 reasons for this:
1) The album wasn't initially mastered through a full digital chain (I have CDs till very late 80's that came out as AAD) so for quality and / or to take advantage of the new technology albums get re-released. These may be worth it if the upgrade is real or one should stay the hell away from them if they experience the compression level of today mastering (thank you I'll stick with my analog processed release from the '80's)
2) Second reason - pure economics, micro, macro and anything in between. Albums have a certain shelf life, after that they go out of print (entirely or just in certain markets). To still generate interest in the material new editions are mastered (completists welcome to your deluxe editions) or - the ones I hate the most - here come your Greatest Hits edtions. When the Greatest Hits run out of print we're invited to a new Greatest Hits wave, slightly different than the first and so on. I'm a devoted fan of a certain band, but as much as I like them for decades now, the joke is that there will be at some point a release of "The Best Of The Greatest Hits"!
Going back to create some more custom fields to deal with the madness...