They pay by giving you greater control of your epoxies properties and increasing the volume of your epoxy at a generally cheaper cost.
There are a variety of types of fillers all with specific instances to use each type of filler.
Wood Flour
Wood flour is a very clean and fine wood powder manufactured specifically for industrial use. Good as a strong non sagging epoxy thickener for wood toned glues and fillets. While the color will darken when used with epoxy this will help blend your gluing in wood projects.
Fumed Silica
Fumed silica known as Carbosil or aerosil is the most commonly used filler and makes a very smooth strong non sagging putty and is often added to other fillers to improve their properties. Silica added to your epoxies will have a white appearance but will be rock hard. It is recommended to get it put on as smooth as possible because it will be very difficult to sand.
Glass Bubbles
Microscopic glass spheres or bubbles are used primarily in fairing a project. These little glass bubbles are hollow so they are extremely easy to sand and get that nice smooth showroom finish. Glass bubbles added to your epoxies will give it a slightly off white color when cured.
Phenolic Micro-balloons
Phenolic micro-balloons are hollow reddish brown spheres that make excellent light weight putties for fairing. They sand easier, smoother and have better sag resistant properties than the glass bubbles. Phenolic is the best quality filler for an easy sanding fairing putty.
Graphite
Graphite powder Black fine powder, used to create slick abrasion resistant putties and as a black coloring agent.
Milled Fiber
Milled fiber is a dense very high strength powdered fiberglass filler. For high tensile strength putties and gap filling.