There is not a single product that softens leather.
Let me explain :
Think of this in terms of the leather fibers – kind-of like the fingers of your two hands interlaced.
When you dye the leather and while the leather fibers are limp, the leather is pliable. Same goes for getting the leather wet for tooling or wet forming.
Now as the leather dries, the dye, and to a lesser extent the water, makes the leather fibers sticky and when it is dry, the leather feels stiff. Many people confuse this stiffness with “casing and/or dying leather dries it out”.
Now you put a sealer or conditioner on and nothing changes. The fibers still stick to each other because of the dye, and maybe now also the sealer.
BUT, as soon as you start manipulating and bending the leather, the fibers break free of each other and the leather becomes softer. If you had applied a conditioner like dubbin or Aussie or neatsfoot oil, the fibers that break free from each other, get lubricated and the leather feels even softer because the fibers now also get lubricated.
I hope this helps!