因为出口企业只有将外汇兑换成本币才能在国内使用呀

主要是中国实行外汇管制 强制结汇制度

以上说的都挺对，简单讲这是实行有管理的固定汇率制决定的。中央银行在这种制度中充当一个“中介”的作用，进口必须用本国货币到中央银行兑换成外币购买外国的商品和服务，出口必须把赚的外币兑经中央银行换成本国货币。所以在我国当前贸易顺差不断增加的情况下，要维持固定汇率制，就必须不停地用本国货币来兑换赚得的外国货币，这也就形成了两种情况，一是外汇储备不断增加，一是我国中央银行不断用本币对冲！总之，两点，我国出口强劲和固定汇率制决定的！

The large amount of foreign reserve in our country is mainly caused by the (managed) fixed exchange rate system which at present set at a level below the equilibrium exchange rate. As a result, the supply of the RMB is smaller than the demand for RMB. The shortage of RMB supply occurs and gives pressure on the appreciation of the Chinese Yuan. However, to fix the currency at the present exchange rate, the central bank has to find means to create the artificial equilibirium in way of purchasing other currencies with RMB. The common way is to purchase Treasury bonds and increase the value of foreign assets held. That in turn leads to the increase of the money supply.

当外汇过多,引起本币升值,就会增加进口,抑制出口.从单个经济体来看，当然不希望出口受到抑制.

发行本币,维持汇率,可以保持国际贸易间原有的状况，不出现大的波动.