Vigor routers equipped with Wireless LAN facilities are supplied with the standard 2 or 3dBi gain aerials. A higher gain aerial provides a signal gain which can in many circumstances improve your wireless coverage/range as well as your connection speed at longer ranges, particularly where your required coverage area is currently borderline and needs a 'little extra help'.
Uni(single)directional aerials are intended for application where you want the signal in one direction only, either for increased range or for security reasons. By concentrating an aerial in one direction, both transmission strength and receive sensitivity are improved as well as benefits in avoiding interference or traffic from adjacent networks in other directions.
Wireless antenna gain is a complex area and we'd recommend seeking out a specialist site for further details but put simply, aerials are designed in such a way that they can increase the signal output from the transmitter's original output.