The fixed base of a router does not allow you to use the router as a plunge router by itself. This means that you cannot use the router to cut designs such as you might see on a cabinet face. The fixed router's stable base is perfect for creating precise cuts around the edge of a piece of wood, however.
Adjustable base, or plunge, routers have a spring-loaded base platform that is attached to a standard router base. This allows you to create decorative cabinet faces or other designs in wood surfaces that do not fully penetrate to the back side of the wood. While there are dedicated plunge routers on the market, utilizing a standard base router with an add-on plunge base increases the usefulness of the router and allows you to use one tool for a variety of purposes.
Because fixed-base routers have fewer moving parts, they are considerably less expensive than plunge routers. Utilizing a fixed-base router with an add-on plunge router base will save you money over the purchase of two distinct tools, as the add-on base is less expensive than the complete plunge router.
Fixed base routers can be used with a router table base, whereas dedicated plunge routers cannot. The router mounts to the underside of the table so that the router blade extends up through the table. Plunge routers cannot be mounted to a fixed base.