Yes it is possible to connect a Sabertooth to a Raspberry Pi. Just set the Sabertooth to serial mode using its dip switches and send serial packets from the Pi. You will need to write this code yourself.
As for whether you need a Sabertooth with the Pi, the answer is it depends on what you’re trying to do. If you need to control motors then you need some sort of motor controller (such as the Sabertooth) since you won’t be able to output enough current directly from the Pi’s GPIO to spin the motors.