Spotify have set up an agreement with Universal Music Group where Spotify's Premium users will have access to listen to artists new albums two weeks earlier than users that are currently using the free version.
A lot of artists work under the UMG brand from Avicii to Robbie Williams, so you'll be sure that Spotify will release your favorite artist's album. They work with a lot of people so potentially a lot of early album releases that you may end up missing out on if you're not a Premium member.
Spotify CEO Daniel Ek stated that “We know that not every album by every artist should be released the same way, and we’ve worked hard with UMG to develop a new, flexible release policy”. Ek declared “Starting today, Universal artists can choose to release new albums on premium only for two weeks, offering subscribers an earlier chance to explore the complete creative work, while the singles are available across Spotify for all our listeners to enjoy.”
The early release agreement with UMG doesn't apply to singles as free and Premium users can access them as soon as they're released; the agreement with UMG is purely albums only.
So, if your chosen artist has a new album coming out soon you might want to consider signing up for Spotify Premium otherwise, you may be waiting a few weeks to hear your new favorite tracks.