They are completely independent of SN1A measurements. They help to measure DE by measuring the rate of expansion over time.
They're better primarily because there are huge numbers of galaxies out there, but only so many supernovae. Also, each individual supernova has a lot of error.
This isn't to say that supernova measurements are worthless. SN and BAO measurements have very different error properties, so that the combination of the two is significantly better than either one by itself.