Buffer capacity as normally used in photography is used with respect to developers for the most part. It is also important in fix baths and stop baths, but not nearly so as in developers.
In emulsion making it becomes important when scaling. The smaller kettles have less "buffer capacity" which in this case isn't related to acid-base reactions but to the inertia or resistance to vAg (pAg) change. The bigger the make, the larger the inertia of the bigger kettle.