To me the meaning of "computing buffer sizes" is unclear. The buffer of what? How do manually entered numbers help to compute the size of a buffer? The rest of the description seems to suggest that this combinator automates the conversion from stack-count to item-count via its UI (or vice versa?).

I have clarified the Information page with the intended usage; let me know if this makes it more obvious

You enter an item and the amount of stacks and it outputs the resulting item count. This is (was) nontrivial as items have different stack sizes and solving this problem was relatively UPS intensive or annoying, by looking up the stack size of each item and calculating it manually.
@Puppy I think you should mention that this combinator just takes the stack size of the selected item and multiplies it with the wanted amount of item slots and outputs the result like a constant combinator. Your description sounds a bit more complicated than it actually is.
BTW, the LTN combinator mod does something very similar, where you can also specify an item and a stack count.

@brainfart with LTNC you can't enter stacks until after you have picked the item. With the Buffer Combinator you can set the stacks, save it, and then the stack size is recorded in the blueprint you are using. The same applies to fluid buffer size; you can set one fluid buffer size in the combinator, then blueprint it, and then the fluid buffer size is remembered every time you place the blueprint. In these cases you just pick the item after placing the blueprint and it is calculated for you. This makes it smoother to use than LTNC.