Accidents happen at very inconvenient times.  Assume someone had a hard fall and is unconciously floating downstream.  Time is of the essence before he gets awaty from you.  Throwing him a rope does no good, you need to swim to retrieve and trust your paddling buddies to toss you a rope to swing back in (or swim the person to safety).  Rescuing gear sometimes requires us to venture into challenging locations.  Quickly set-up a tethered rescue and swim wqith a fair amount of confidence to the pinned boat or paddle.  Rescue tethers can also be used for lowering on the water surface to get to the accident site.  Rescue tethers are great assists in marginal/slippery foot environments as well.