Because you need to use proprietary graphics drivers for acceleration. Another reason could be that you want to write more portable code that can run in your RPi during the early stages of development and on a board with no GPU later on.
Actually, you don't. Raspberry Pi is still a proprietary platform (you need a closed firmware blob on your SD card for it to boot), but open GPU driver is now in Mesa and in some cases works better than the old proprietary one.