Essentially, the summary of this article is that this dude thinks he can leverage these song catalogs better than other investors. He's literally a music fund manager for music.
His firm is "overpaying" for these back catalogs because he's essentially putting together a collection that might command more revenue than other "fund" managers, and possibly set up something of a targeted monopoly, and he's betting that they can extract more value than less competent managers and record labels.
It wasn't directly addressed in the article but I suspect that the general strategy is to own the majority the sorts of songs that are more suitable for generating Synch fees. A big corporation is going to look around for a collection of songs that tend to do well in a TV commercial, for example. Perhaps they find that Hipgnosis happens to own all of them, and now they have to pay a higher rate, and all of a sudden the high price paid for this catalog doesn't seem like overpaying.
