A router can greatly speed up the process of preparing edges for joining, however much effort can also be wasted unnecessarily. As has been stated, a simple glued joint is very strong, and is sufficient in most situations.
Where a router excels, though, is for creating interlocking joints which also increase the wood surface to be glued.
These router bits give a simple tongue-and-groove style interlocking edge that increases the gluing surface and when the boards are mated together, positively align the boards to be glued. These bits are only applicable when the edges to be joined are exactly the same thickness. With the router bit adjusted to the correct height, first rout one of the edges face up, then rout the second panel with the face down. The pieces can now be joined edge-to-edge with both faces in the same direction.
These router bits produce a joint that is similar to the glue joint, however it has many more projections. It is produced in the same way as glue joints are. The advantages of finger joints is that the surface area in the joint is increased, increasing the gluing surface.
A variety of router bits are available that produce with one (sometimes two) passes through the router, a joint that locks together with another edge, giving a more mechanical connection that a simple glued joint could provide. These joints are often used for edge-to-face joints or edge-to-edge joints. The edge-to-edge joints are produced in the same manner as glue joints above, however the edge-to-face joint has the second piece routed in a vertical position, with the profile being produced on the board's face instead of the edge. Care should be taken to correctly orient the boards before routing.