PCI graphics cards were popular up until the late 90s, when they were superseded by AGP, which was a dedicated graphics card slot with DMA access to system memory. PCI cards depend upon a low speed bus to receive data. read more
The graphics card were designed in such a way that these type of card could be inserted into the PCI slot or else in the AGP slot. Because the graphics card in olden days were itself not so fast enough for graphics acceleration and color generation. read more
In the event that one of the PCI Express slots fails, you can move the graphics card to the secondary slot. The secondary slot will still be influenced by BIOS and jumper settings even if the primary slot is dead. This can be a problem if the primary slot is down and you can't get into the BIOS to adjust the settings. read more