The anchor straps are needed to properly install Stretch in your car. They are attached to the floor of your car, so that you can attach the retracting lower tethers to the anchorage straps.
 
One pair of anchor straps is included with the seat. If you have several cars it might be convenient with an extra pair of anchor straps so you don’t have to de-install when moving the seat between cars. Buy an extra pair if needed.