It is used because it is specifying which states. It is not just some states in general, but the United States of America. People may also say "the States" or "the US" or "the USA" for short. Also for the United Kingdom / the UK you must use the definite article, because it is not just any kingdom, it is the United Kingdom. A specific kingdom. Also many islands require the definite article. The Philippines, the Galapagos. That is because it is a short form for "The Galapagos Islands". You are not talking about any islands, but about these ones in particular. I hope this helps : )