It doesn’t necessarily “help” form, but it takes the front delts out of the curl and makes the movement strictly elbow flexion. It’s not better, it’s just different. Because the biceps also play a role in shoulder flexion/raising the shoulder, the arm blaster takes that part of the movement out of the equation and changes the contraction. Thibaudeau talked more about that here.
I can’t find the pic, but I’m 90% sure that way back in the day there was an early version of the “arm blaster” that was basically straight across and didn’t have grooves for the arms to settle into (imagine like a 2x4 with a neck strap), so the elbows were slightly in front of the body. Again, it’s not better or worse. It’s just a variation. We’re talking curls, not astrophysics. The details don’t matter all that much in the big picture.