Steel shot loads performance.

[Mr. 16 gauge]

Steel does not deform and the reason for buffer is to help it be 'more fluid', as skeet has stated......remember those toys that had 4 metal ball bearings on a string, and when you smacked one, the one on the other end would jump up? Well, steel shot does the same thing when fired....energy can be transfered from one pellet to the next, opening patterns. Buffer helps to 'cushion' the steel and prevent this, tightening patterns.
With bismuth shot, the buffer is used to help prevent the fracturing of the shot.....kind of interesting to pattern a bismuth load and find more holes in the target than there are pellets in the load!
I use buffer in my bismuth loads and some of my lead loads.....I worked for two years to get a decent 16 gauge turkey load, and in the end I had to use buffer to help tighten up the patterns. Would have been great if there was a factory load that offered it (tighter patterns), but since the majority of ammo manufacturers ignore the 16 gauge, it was a bit of necessity on my part........and, btw, I did enjoy the challenge of it all.