So part of it was to be backwards compatible with some mono record players, that might be the needle explanation. Another part was how the mixer wanted to use the two channels. Are the channels left and right, or two mono channels of vocals and instrumentals. Lastly it was because mixers could do that when just a short time before they couldn't. They were artists who were experimenting with a new medium. It wasn't just one reason, and it wasn't for very long really. It's just a lot of music was recorded during that time.