It uses larger memory chips, so you use less chips on a stick to get the same amount of memory, thus saving a bit on manufacturing costs. It generally doesn't affect performance (assuming the memory timings are the same between the high and low density sticks), but it can potentially affect the RAM's compatability with some mother boards. I think it was mainly just an issue with high density SDRAM DIMMS on certain older boards, though, I haven't heard of any modern boards that have major issues with high density DDR DIMMS.