I've noticed several of the later hits the group had have different vocal and even different versions of the songs. But songs like Where did our love go, Lovelight and Come see about me, were they just one vocal take songs? Anything good lie beyond the fade?
supposedly the artists at motown would record multiple takes but then only keep the "good" one. HDH have said the girls were fast in the studio and would do things in only a couple takes, at most.
But once they were done with the song, the other versions and takes were often erased so that they could reuse the tape for another song.
now why some are still around, i don't know. and why only certain songs or more of the later ones.
In the case of "Where Did Our Love Go" it was at least a partial alternate based on the legendary story told, where Diana attempts to sing the song one way but it's not what HDH wanted. Legend has it that the record we've all come to know and love was the second take. Now whether the first take still exists or, to Sup's point, erased to make room, is anyone's guess.
Some recordings were dubbed onto an acetate at the time and used for reference purposes and it is those that would be the most sought after in 2019. In the 60's, good quality recording tape was expensive and recordings were often wiped and re-used for other recordings, as Sup_fan stated above.