It's a way to keep more Steem locked up. Free Steem can be sold, and since Steem is continuously generated, there would be plenty on the market. This leads to sellers tending to lower the price to sell it. Consequently, the token's value drops. Locked Steem cannot be sold until we unlock it, so the more Steem locked in Steem Power, the less Steem supply there will be on the market, and consequently, resistance to the progressive decrease in its value.