In order to represent the scope and inventiveness of the Revolver album, the Beatles chose to release a double-A side single with two of the most diametrically opposed songs they ever bundled together - and since that was a move they often pulled that's saying something. While both tunes are illustrations, one is decidedly escapist and the other grounded in a rather harsh version of reality.
"Eleanor Rigby" is brilliant in just about every way it can be. It begins almost in medias res with the enigmatic chorus phrase "Ah, look at all the lonely people", while a string quartet swirls belligerently around the vocals. It wasn't the first time strings were used on a Beatles songs and not the last, but while "Yesterday" was meant for a languid beauty, "Rigby" has them tensing the situation up, wrenching pity out of the listener for the subject of the song. The details themselves are extremely well chosen, using only a few tableaux to draw Eleanor and Father McKenzie together without saying why they are inhabiting the same song, besides...
All the lonely people,
Where do they all come from?
All the lonely people,
Where do they all belong?
That. That fucking chorus. Man. That's a sentiment you simply would not expect out of the Beatles... ever. They're a Greek chorus, trapped outside the narrative. This is such an economical, load-bearing phrase. It sums up the song but instead of resolving it, deepens it by drawing up more metaphysical questions. Ambiguity was staking more and more of a claim in the Beatles' lyrics and music as of Revolver, as distinguished from vagueness by details and implications like the ones found in this song.
By contrast "Yellow Submarine" is an experiment of its own, and though it's more technicolor kiddie fantasy it's no less worth noticing. For the burgeoning psychedelic era, childhood imagery was a great tease, both an escape and a highlighting of how dire things were in the real world. It's quite tempting to run away and go "Live a life of ease" where "Every one of us has all we need," especially since that turns out to be "Sky of blue and sea of green." For the next while, the Beatles' music represented a passageway to the ordinary turned extraordinary, and the joys of a simpler existence. Real, earnest hippie stuff that doesn't seem so dated today.
As for the song itself, it works. As corny as I may sometimes find it, it hangs together as a kids song for adults, memorable and fun, with a thrown-together-in-the-basement, sound effect-laden, just for laughs feel.
Seriously, the contrast between the artfully baroque, existential craft of "Eleanor Rigby" and the anything-goes family friendly feeling of "Yellow Submarine" must have made for an irresistible teaser for Revolver when it was simultaneously released with this .45 in August 1966. How could you not want to know what else existed between these two signposts of advancing ambition, exploration, and plain old quality? Maybe there were better or more commercial songs on the album, but few paired together would have summed it up so succinctly. The dark and the light.