Hence there is immanent in capital an inclination and constant tendency, to heighten the productiveness of labour, in order to cheapen commodities, and by such cheapening to cheapen the labourer himself. (1)That is to say, a capitalist strives to increase the amount of surplus value (profit) he makes.
This surplus value is the excess hours worked beyond the point where a worker has created enough value to maintain himself. The capitalist thrives either by extending the working day, or, if that is not possible, by reducing the cost of living which the worker must recoup before creating surplus value. This is done by driving down the value of the commodities the worker needs to maintain themselves.
Hence, the delight of cheap imports. They reduce the cost of living for those still working in more developed economies, exerting downward pressure on wages.
1 - From Capital, by Karl Marx. The quoted passage occurs in Chapter 12, 'The Concept of Relative Surplus Value.'(http://www.marxists.org/archive/marx/works/1867-c1/ch12.htm)