Tomorrow, the indie favourite Conor Oberst releases the latest Bright Eyes album — though Noise Floor is not a new album, as such. It is subtitled “Rarities: 1998-2005”. Next month, Tom Waits releases Orphans, a three-CD set that mixes new songs with a collection of 26 rarities. And only a couple of weeks back, a new Byrds retrospective enticed fans of the 1960s folk-rock pioneers with rare tracks recorded by band members before they had even settled on their name.

You might be forgiven for thinking that in the age of digital downloads, when all the music ever made is surely available somewhere, there really is no such thing as a “rarity” any more, that the rarities album, or the reissue bolstered with rarities — whether B-sides, outtakes, demos or live versions — is actually something of an anachronism. But, judging by the release schedules, both artists and record companies disagree. The “odds and sods” album seems to be alive and well.

Cynics say that this is because record companies see them as an easy way to make money: low-cost (the material already exists) and easily targeted to an existing fan base. Perhaps this is true in the case of the Byrds box set, but look at the other artists I’ve mentioned. Are these people we expect to make cynical moves to squeeze money out of their fans? Hardly — Waits won’t allow his music to be used in commercials.

So, clearly, these albums are important to artists in other ways. There is a clue in the title of the Waits album. Orphans was originally envisaged as purely a rarities album — the decision to add new material was made later. Here is how Waits describes the rare tracks: “Scared, mean orphan songs of rapture and melancholy. Songs that grew up hard. Songs of dubious origin rescued from cruel fate and now left wanting only to be cared for. Show that you are not afraid and take them home. They don’t bite, they just need attention.” Even allowing for Waits’s picturesque way with words, we get the point. As far as Waits is concerned, these songs were in danger of being lost. Collecting them together means they have a better chance of garnering the attention he believes they deserve.

Even in the digital age, artists have a real fear that some of their music can get lost. If you saw the haphazardly arranged piles of tapes and discs that line their studios and homes, you would understand why. As Waits puts it: “Gathering all this material together was like rounding up chickens at the beach. It’s not like you go into a vault and check out what you need. Most of it was lost or buried under the house. Some of the tapes I had to pay ransom for to a plumber in Russia.”

Waits’s rarities are culled largely from collaborations and work for other people’s projects (such as his recording of Heigh-Ho, the Seven Dwarves’ song, for Hal Wilner’s Disney album, Stay Awake). Orphans contains no B-sides or alternative takes; but there are plenty of both on the recent reissue series of Elvis Costello albums, each of which comes top-heavy with rarities. These exemplary reissues, which also contain extensive and deliciously honest liner notes by Costello (“I hated just about everything in my world, reserving the greatest disdain for myself”), reveal another reason why artists need to collect and organise their rare tracks. Some songwriters are just extraordinarily prolific: they write far more songs than a conventional release schedule can cope with, then record these songs in many different versions and styles. Thus, on the Costello reissues, the original tracks are far outweighed by the bonus tracks. On Get Happy, for example, where the original boasted an already impressive 20 songs, the reissue flaunts 50.

A rarities album can also be useful at a point when a band isn’t being prolific enough. The wonderful power-pop band Fountains of Wayne had something of a commercial breakthrough with their 2003 album, Welcome Interstate Managers, but, two years later, hadn’t put together a follow-up. Never mind: a two-disc rarities set, Out-of-State Plates, kept their new fans interested. When the Cowboy Junkies issued their album Rarities, B-Sides and Slow Sad Waltzes, their singer, Margo Timmins, described it as “a gift” for fans. Not that they were giving it away free, you understand; Timmins’ point was that “as a fan of other bands, this is the kind of thing I really like — like a little treasure. So that’s why we put it together”.

Even in this harsh commercial world, it’s still possible to think like this. Depeche Mode’s core fans will treasure the re-releases of their albums, which come not only with rarities attached, but with accompanying DVDs of live performances; Van Morrison fans had waited patiently for years for the unreleased material that emerged on his Philosopher’s Stone double album.

The digital age seems, in fact, to have breathed new life into the idea. Although the internet makes music more freely available, it also fuels the kind of fervent fan chatter about lost recordings and alternate versions that creates the market for rarities albums. Plenty of attention has been given to the fact that our download age makes it easy to cherry-pick an artist’s best few tracks; equally, a life spent trawling the online world surely encourages the kind of completist mentality that makes Orphans or Noise Floor a widely anticipated treat. It is clear that the rarities album isn’t in danger of becoming a rarity.

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