if yours broke down
if you want to upgrade and what you have is either too old or does not support the cpu that you want
2cents, build quality, overclocking potential (though it varies a bit), board layout (position of ports, etc)
overall looks, price (i dont buy cheap boards i just buy mid range ones), color (if you want a theme),
number of pci-e (if you plan to go sli/xfire),