A Safety Strap is a nifty little piece of gear that increases the number of connections with your dog. By clipping your lead to the D ring and then to their harness and collar or by connecting it to the lead, you can ensure that should the worst happen and your dog slip their collar, you still have hold of them.
 
This makes it an important tool when walking unknown dogs, rescue dogs or those that pose a flight risk. 
 
It also makes for a useful grab handle when connected to both the collar and harness as well as taking some of the worry away when swapping from a lead to a long line.