Higher transfer speed. Depending on the bus it can cause a serious slow down. If the bus on the circuit board or travels a long distance, then transfer speeds are in the MHz not GHz. So to move more data they need more lanes. This is kind of like parallel vs serial.
It is only the address bus width of the processor that is limited to 64-bit but not the data bus width. For example, Cortex A8 has support for 128 bit data bus. Refer to the below link for details.