Why do some MB have 2 or more PCI-E Slot?

Some motherboards have two PCI-E slots mainly to take advantage of Crossfire or SLI, which pairs up two similar GPUs together to give a boost in performance.
Heres a link explaining ATI's Crossfire. Nvidia's counterpart is called SLI. (Note that SLI originally stood for Scan-Line Interleave, which was a technology made by 3dfx's Voodoo line to boost performance by linking two cards). Remember that SLI only works on a motherboard that supports SLI, and likewise Crossfire only works on a motherboard that supports Crossfire. For example, Crossfire is supported by the ASUS Rampage formula motherboard, but SLI isn't.

Under TH's graphics charts section, you can find several benchmarks showing the results of dual card vs. single card configurations. Note that SLI and Crossfire scale better at higher resolutions, and sometimes loses performance against single cards at lower resolutions.

this is as far a the best knowledge i've got since the birth of video game cards. thank you so much.

so if you have a motherboard with SLI or Xfire enabled, you can pair up 2 or more Video cards to act as one video card to boost the game gfx intensity? wow. im so leaving on rocks. i thought having a Ati Radeon 4850 will do the trick on assassin's creed and crysis warhead.