Well, i see some reasons:
1) people expect it to achieve greater value in the future
2) you might want to make it circulate so that it makes bitCoin an important coin, making it last forever
3) you might be from some other country and might want to by something from accross the sea ... instead of buyin dollars, you can buy bit coins (that's what i am gonna do)
~~~~~~~~~~~~~~~~~~In that case when I sell them Bitcoin, I got their money, I will have to transfer to USD my self?
4) this might be your business, to buy bitcoins to sell them to other people who need/want them (for any reason)
Well, not exactly ... if i find someone in my country who deal with foreign bitcoin seller, he will buy, so he will transfer the USD, then i will buy from him using my countrie's money, then i can buy from any store across the sea without ever buying dollars.
If bit coin really catches on on every country, that store can buy something accross seas again from my country, therefore, there will be, again, bitcoins here so that i can purchase (or trade for goods and services)