Tow rope tied to the small motorboat stored inside the ship, the purpose is that when the motorboat is lowered and hits the water, it will move along at the same stable speed as the destroyer. The boat crew will release the cable before they move away.
It's not necessarily to keep the RHIB moving along at the same stable speed as the destroyer, but rather to help provide an additional point of stability as it is lowered down, and to act as a mooring line for the RHIB to stay alongside the destroyer to allow the RHIB crew to embark/disembark.
A destroyer generally wouldn't be moving too fast, if at all when deploying a RHIB.
Also, the situations where you'd need to deploy a RHIB would rarely require your mothership to be moving underway too fast either.
These two videos are good examples of the process
.... that said, I'm also not sure why they would need a mooring line for RHIB deployment that far from the bow, they could just deploy those mooring lines from the 055's davit without issue.