more dipoles increases the overall antenna gain. they are usually done in what is called a phased array.
a bunch of antennas gives good gain and is much much cheaper than an equilivent amplifier of the same gain!!
as a fellow ham radio friend of mine likes to say "gotta love aluminium amplifiers"
also to improving the gain of the system cheaply, when using a bunch of antennas like that it is easy to adjust the radiation patten of the antenna system so that it can be directed to cover certain areas more efficiently
cheers
Dave