Mainly for gamers, one gfx card is not enough. They need a second one. Sometimes third one, fourth one.
For other people, that PCIe x16 slot can be used for any other PCIe card, x1, x4, x8, and x16. You can use sound card, ethernet card, etc. in that slot. In some cases, using that slot may lower bandwidth for the first PCIe x16 slot. Check the motherboard manual.
for exmaple if i have 2 graphic cards each of 1 gb.after installation total graphic memory will be 2 gb?
Yes if you have 2x 1Gb gfx cards, your system has 2Gb of ram to use onboard the cards for processing your pretties, but they must be linked together to work together,
Nvidia use SLI technology (Scalable link interface) and ATI cards use Crossfire,
you will need cables to join the cards together so they 'talk' and share resources
mainly its hardcore gamers or highend design users that would benefit from using Sli or Xfire, but its recently been drifiting into the mainstream as well