While good cementitious grout has its benefits, the capillary pores in the grout mortar mean that dirt and other external elements could find their way into the grout and becomes trapped / embedded on the surface. This is where stains occur and sometimes mould and mildew could develop. Cementitious grout is generally not a waterproof barrier, though the addition of a liquid latex additive for the grout mix or a grout sealer can help to further improve its physical properties.
Epoxy grout, on the other hand, is made from epoxy based resins and filler. The components that made up epoxy grouts make it waterproof, and possess greater physical properties, such as chemical and stain resistance.
Though epoxy grouts cost much more than cementitious grout, they are durable and in the long run more cost efficient and value for quality.