I suppose if one has to make an inexpensive product to compete in the marketplace, then fillers are inevitable to cut the cost. But they are not necessary for the function of the soap, and I think fillers have little or no place in well-made handcrafted soap. My opinion here -- YMMV.
Anything intentionally added to soap that reduces cost but doesn't add much or anything to the function of the soap. It can be too much of a normal ingredient. For example, an excessive amount of water can be thought of as a filler. An overabundance of washing soda or borax can be filler. Other things that are obvious fillers include starches, wood fiber, clays and other mined minerals like talc, sodium silicate (water glass), etc. Some of these are/were used because they allowed extra water to be added to the soap without greatly affecting the appearance. In the day when rosin (colophony) was cheap, a modest amount was often used to add detergency to a household or laundry soap, but when used in larger amounts it was essentially a filler.