Fence tension wire is an important add-on for pet fencing.
For one, if posts are positioned closer than the recommended 15-20' feet apart, the fence mesh will sag, thus putting jumping dogs at risk of getting out [and external wildlife getting in]. Monofilament wire keeps the fence nice and taut and will be appreciated by homeowners in high wind areas and with trees around the property. Should a fallen branch land on the fence, the tension wire will keep the fence strong.
When considering add-ons for dog fence, look into dog fence tension wire.