its an option for people who already have a gpu, but instead of upgrading the gpu, you buy the same gpu that you own to cut price for performance. though doing so is heavy on the power consumption of your computer. usually if your computer is a first build, you will only have 1 gpu unless your sli/crossfiring the top of the line cards
For example
Say I have a 6850(150$) and i want to upgrade my GPU
I can pay 250$ to upgrade it to a 7850
BUT
I have the choice to buy another 6850(150$) and crossfire both to have good performance without having to spend 100$ more to upgrade my graphics power