^It makes them more rare. It raises the value of the BTC that already exist. If there were no rarity to Bitcoins, and we just kept "printing money," bitcoins would be like the US Dollar. They have to become harder to get, so their value goes up. People see that as a potential investment, and buy bitcoins.
In practice, I don't think halving the reward does anything to the value of the BTC that already exist. The only thing that halving the reward does is compensate the miner less for his (increasing) amount of work he has done.
I think the "value" of the BTC is controlled only by the human beings who hold those BTC, and those who don't hold them but would like to. Supply and demand.
Maybe this issue is addressed when the blocks come with "fees" in addition to the "reward", but nobody is really talking about that yet.