With some classical labels, such as 1970's DG, original vinyl issues were pretty poor in terms of sound quality, but the label took advantage of the opportunity to remaster for digital and put out some pretty decent reissues. Other labels, such as EMI, are all over the map in terms of sound quality; some reissues of their great sounding vinyl releases sound like crap, while certain other reissues sound pretty good.
Certain reissue labels, such as Brilliant, make it easy to build a comprehensive collection of a particular composer's works (e.g., the massive complete works of J.S. Bach); one of my favorite Brilliant collections is the complete string quartets of Villa-Lobos (something I probably would not have even looked at, except that Brilliant put together a very cheap and easy to acquire collection). CD's have been such a boon to collectors of both older material as reissues, and to collectors of newer releases (It is far cheaper to make CDs and digital downloads than to press records; given how poorly classical sells these days, almost nothing could be economically released if vinyl were the only medium available).