Not all embedded boards are supported in Main Kernel Tree. For those boards, mostly manufactures write the code required for linux kernel to talk to on board chips and peripherals.
BSP includes cross compiling tool-chain for your target as well, I believe.
BSP also provide interface for initialising processor and exposed various other board features to linux kernel. BSP also provides all the device drivers for board which are not supported by main kernel tree.
In nutshell, if you can't find your board in main kernel tree, then you most probably need BSP for your board.