"MacArthur Park" has been recorded by everybody from Donna Summer to the Four Tops to Frank Sinatra. It was written by Jimmy Webb about the end of his relationship with Susan Ronstadt, sister of Linda. The song was originally part of a longer piece that ran for 22 minutes, which is why the Association turned it down.
Richard Harris chopped the track down to seven minutes and released it as a single in 1968. He screwed up by repeatedly singing "MacArthur's Park," a mistake that was later copied by Donna Summer in her famous remake. Yes, it's a little stupid that he uses the analogy of a cake out in the rain to describe his ruined relationship, but it's not a terrible song.
Case in point: check out Weird Al Yankovic's "Jurassic Park" parody. It's an under-appreciated classic. Sample lyric: "Jurassic Park is frightening in the dark/All the dinosaurs are running wild/Someone shut the fence off in the rain/I admit it's kind of eerie/But this proves my chaos theory."