It’s old insulation tape to prevent the pipe from freezing, looks like the backflow preventer has seen better days, plus they are using old fashion gate valves for shut offs instead of ball valves. Need a insulated sock cover that way it’s covered clear to the ground and below if they want to.
I think it’s duct tape that’s been beat up by the UV over the years. It’s not for leak prevention, it’s for frost protection. There’s insulation under the tape. The modern slip-on insulated covers are superior and reusable but this could easily be older than them.
Keeps the pipe from sweating when cold water is in it, which prevents the pipe from rusting. Or it used to. Not enough insulation to prevent any kind of freezing or anything like that, those educated guesses are wrong.