PCI Express data transfer method? Serial VS Parallel
It's 32x faster.

Graphics cards require lots of high-speed data transfer, so they get lots of lanes; other devices like sound cards and serial interfaces can be served adequately by a single lane, which is cheaper.

In response to your edit:


  Sure, 32 lanes means 32 times faster, but then you can't say the communication is serial, because by definition it means that each clock cycle transfers one bit of information, whereas in PCIe x32, 32 bits are transferred at each clock cycle. 


Not quite: the lanes aren't synchronized with each other! That is the key difference between multiple serial lanes and a parallel interface. Each lane has its own clock. Even if multiple lanes transmit in a synchronous fashion, because the lanes aren't required to be exactly the same length they may recieve asynchronously.

https://en.wikipedia.org/wiki/PCI_Express#Serial_bus explains this.

(I'm trying to find a good explanation of whether bytes are split across the available lanes before or after the 8b/10b encoding is applied, but this detail of the physical layer seems to be underexplained)