|k-pop girl band f(x) is hard enough to search for already because of their name,
but the fact that their music keeps disappearing from Spotify's catalog
for legal reasons makes them even harder to find.
The reason things like this keep happening is that the legal rights to stream music are a total mess. Some tracks are licensed through what the Copyright Office calls a compulsory license. Others are licensed through a third parties that act as a licensing clearinghouse. Big players like Spotify can cut deals directly with record labels. It's possible that in the future we could have online music providers going straight to the artists themselves, perhaps to get exclusive content. At any given in point in time, someone can fuck up the copyright licensing and inspire an artist or label to sue, the streaming provider could decide not to renew a license deal, and so forth.
This is a long-winded of saying that even with the advance of streaming services for music (Spotify) and video (Netflix), there are still good reasons to buy MP3s and physical media such as CDs and Blu-rays. A Blu-ray disk sitting in your closet has a much longer shelf life than Netflix's current contract with Disney, and if you think that five years down the road you might want to dig The Avengers five years from now and watch it while on staycation, you'll be better off owning the movie in some way than relying on to keep their licensing agreements up to date.