For record labels, the question is whether cooperation with Spotify provides consistent returns. (The issue came to the fore recently when a U.K. distributor decided it didn’t and yanked more than 200 labels from the service.) For me, a user, the question is how useful Spotify really is.

To recap, Spotify is a legal, all-you-can-listen music service with access to some 15 million songs. Music is beamed from the cloud to users’ computers or mobile devices. It works and it works well. In the U.S., there are three pricing tiers: a free ad-supported service that places a listening cap — 10 hours and 5 plays per track each month — after six months, a $4.99 ad-free version and a $9.99 plan that adds a few extras like a mobile app. CEO Daniel Ek is betting that by giving users a free taste for long enough, they’ll get hooked and buy in.

At an upscale event in New York yesterday, the company confirmed its worst-kept secret: apps are coming to the service. Like nearly everything else in techland, Spotify is becoming a platform upon which third-parties can create HTML5-based apps, much like Facebook and Apple (AAPL). The goal? Create an ecosystem that expands Spotify’s reach and makes it more useful.

The move also solves a problem that’s nagged me since signing up: discovering new songs, albums and artists. Ek has in the past argued that Spotify is about putting all of the world’s music is at one’s fingertips. Indeed, with those 15 million available tracks — and 20,000 more being added each day — the service virtually achieves that. But while it can be wildly exciting to search and find again and again, the service quickly overwhelms. Spotify’s vast collection is begging for curation and new ways to find songs that are relevant to a user’s taste.

What Spotify offered before in this area was rather paltry. A feature let users check out Facebook friends’ playlists. There was also a barebones “What’s New” section and two Top Lists — top tracks and top albums — that were the equivalent of Billboard’s Hot 100. (Now with Billboard’s Spotify app, there’s an app for that.) But those limited discovery tools bubbled up a relatively small amount of content.

Why does it even matter? For Spotify to continue to be successful (the company reports 10 million active users in 12 countries, 2.5 million of whom pay), users must be compelled to spend as much time as possible using the service. Not having to look outside for picks from Rolling Stone or Pitchfork, for example, makes that much more likely. And that means more people may find Spotify indispensable and pony up for a paid subscription.

Beta apps I played with from third-parties like Rolling Stone, Last.fm and We Are Hunted seemed promising. Rolling Stone offers playlists from editors and celebrities like “Jagger’s Top Reggae Songs.” Last.fm, meanwhile, cooks up playlists based on users’ listening habits, much like the Genius feature in Apple’s iTunes. Other apps further augment the experience: TuneWiki displays song lyrics in time with the music and Songkick pulls up concert information for acts you like.

If developers buy in, Spotify will have much more going for it than a really big library. It could end up being useful at every phase of the listening experience: discover an artist via Pitchfork app, check out the lyrics, even catch the tour. That sounds like a plan for ruling the music space to me.