Because China is a very large country, they can produce lots of cheap goods. The reason for this is that they have lots of natural resources like wood and metal and also they can employ more people that in Wales because of their huge population.
In addition, we are able to buy a greater amount of different products from China because they can make much more than in Wales. This is because China is larger and has more room for different factories.
Buying from China helps poor people in that country to earn money. Without people in Wales buying Chinese products, those workers would have much less money.