The Beam Reflector is used to extend the range of energy beams. Without using a Beam Reflector, energy beams have a maximum range of 16 blocks. With Beam Reflectors where needed, the beam can be extended indefinitely.
The Beam Reflector can also change the direction of the beam: it will always send the beam to the next Beam Receiver or Beam Reflector in line.
Tip: It is possible to right-click the Beam Reflectors to rotate them to define where the beam will point to.