To my mind there are few reasons that a song must be in a particular key. You might play a nonchromatic instrument and transposition is not possible. Or if a tune of a song is wide ranging, the choice of key becomes important.
An example of the second is The Star Spangled Banner. Common arrangements for bands frequently pitch the song at a point that only Uma Sumac could sing.
You will frequently see arrangements of folk songs in books that are pitched by the publishers arrangers into weird keys (F#m, for instance) that have nothing to singability. I think of this as part of the "Hal Leonard Effect", when a publishers claims (lamely) that any change whatever to a song make the WHOLE WORK rather than just the printed presentation as copyright protected. The claim is then made that any use of the song in F#m must certainly have originated with the publishers version.