DPI and Retina image explanation

All the time, I've been reading about the @2x images behing only image that are twice as big as regular image, but do we have to change the DPI to make the quality better or we only need an image that is twice as big ? I'm sorry if it sounds stupid or anything, but I don't really understand why would a twice as big image of the same quality would be better? Thanks alot
 
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