Torrent sites are also serving this need, but the barriers to entry are high. There's not many (or any) distribution channels for some of this stuff today. If you're looking for some old screamo that was only released in small numbers on vinyl, good luck finding it. That's an extreme example, but really we're talking about preserving art here. Some art has millions of prints, some was one-time performance art. The music industry will only sell you a very small % of what has been created.