I've never understood why 12/8 is a thing. I've never heard a song in 12/8 that didn't just sound like it's in 6 to me.
It has a certain analogy with 4/4 over 2/4, insofar as the first and third beats are stronger than 2 and 4, with 1 being a little stronger still. In 6/8 the implied default pulse of each alternating bar, without further direction, is equal:
In many cases. 12/8 fits the meter and shape of the melody or words better than 6/8.