I am not well-informed about when/where to use 12-degree inserts, but would expect an angle like that for cutting aluminum or plastics, and a still-higher angle for cutting wood. Also, some 12 degree inserts are for parting or roughing mild steel if I understand correctly.
Well ... I don't use inserts very often for parting, but I would suggest that the 12 degree might work well with aluminum, and maybe some plastics up close to where they are held.
And you probably should prefer the 0 degree for parting brass as well -- it tends to self-feed with rake. Same with "dubbing" (removing the rake for a short distance at the end of the drill bit) twist drills to prevent diving through when the hole breaks through.
Mostly -- try and see what work for you, your materials, and your machines.