Many music fans put sleevenotes as one of the reasons they prefer LPs over downloads, streaming, and even CDs when the artwork is shrunk to fit and is too small to read!
But go back to the rapidly expanding commercialism of the 1960s and record sleeves, inner and outer, were considered by some record companies to be fair game to carry advertising. Well there are two 12″ x 12″ boards. Here’s an example of almost three quarters of the back cover devoted to listing other albums. (It looks dirtier than it is).