I2C concept

i2c_register_board_info- first parameter defines the bus number and this should match with the id for the device (which is resident on the I2C bus) while registering in board initialization files. AFAIK this bus number is basically an identifer to distinguish between different I2C bus drivers.Am i right?? If yes then in our platform we have only one I2C bus driver then why do we have to give different bus numbers?? Our devices are connected to different SDA and SCL lines(gpios basically) and for which we are using different bus numbers but we have common I2C driver. So why different bus numbers???