The thing that I've always admired about Burton Lane's music for Finian's Rainbow is the games he plays (probably with some encouragement from lyricist Yip Harburg) with structure. Lane was not Harburg's first choice for the project; they'd worked together on a few songs for movies, but he had done very little stage work -- and would do very little stage work thereafter -- and Harburg would have done the show with Harold Arlen had he been available. But while Lane's music for movie and pop songs had always been inventive and rich-sounding, his Finian songs play all kinds of structural games that go beyond even Arlen; almost none of the songs has a conventional pop-song structure, which at the time was either A-A-B-A or A-B-A-C. The most normally-structured song is "Look To the Rainbow" with its verse/chorus form, but most of the other songs keep piling on new melodies and new sections just where you would expect the opening section to come back in any other song.
So the refrain of "Necessity" essentially doesn't repeat anything except the opening phrase ("Necessity, necessity"). Otherwise it just keeps creating new melodic material, one contrasting section after another; the structure of the refrain is something like A-B-C-D-A-E-F.
And then there's the lyric, which has that special Harburg quality of concreteness, using real, tangible images wherever possible, along with the social commentary that never becomes preachy. (Harburg's attitude in his songs always alternates between arguing a point and ironically re-examining the point he's making; he's like the Shaw of Broadway lyricists.) I especially love this quatrain:
My feet want to dance in the sun,
My head wants to rest in the shade.
The Lord says "go out and have fun,"
But the landlord says "your rent ain't paid."
That's a perfect four-line comedy section, building up to a punchline; it also has specific images of specific actions, contrasting images (dancing, resting; religion, day-to-day reality) and a play on words (Lord, landlord). And it all rhymes, and all sounds like the singer is making it up off the top of her head, when the lyricist probably worked on this for days.
The video doesn't have the verse ("What is the hoax/That just provokes/The folks/They call God's children") but it does have the complete number from the first refrain onward.
The same YouTube channel has also posted the show's biggest hit, "How Are Things In Glocca Morra?" sung by Kate Baldwin (who will be repeating the role on Broadway). Again, the structure is unusual; at the point where a conventionally-structured song might wrap up quickly, it introduces a completely new section ("So I ask each weeping willow") which is almost like a new song-within-a-song.