On a vehicle there will be a fuse on the negative and positive leads for anything not using the chassis as the return/ground/negative lead. Well there are some uses of a positive ground but that's rare outside of telephone gear. Generally the negative post on the battery is grounded to the chassis of a vehicle.
The reason for this was touched on in other answers, the need for two fuses is to cover some known failure modes. In automotive applications the answer is that with a large load, like the starter motor, a failed ground on the large load can find a return path through the vehicle chassis, through the device chassis, down the small ground wire, to the battery. Without a fuse on that negative lead a 500 amp starter could be grounded with a 12 AWG wire to the battery. That wire would then become the fuse and burn up.
Others mentioned the additional hazard of the antenna becoming part of the circuit on any kind of radio. Having fuses on both positive and negative will protect the radio and wires on most failure modes. There's going to be some rare failure modes not covered by the two fuses. For those hopefully a properly installed fuse for that circuit path protects the battery, wires, and people around them.
You are going to get seemingly conflicting information if people are discussing different environments. As a ham radio operator I will wire a DC supply differently at home than in a car. If we look at AC circuitry in a home as an example there is only a circuit breaker on one terminal, the supply or "hot". In fact there is a redundant return path, ground and neutral, to make doubly sure that path is not broken. There's a different philosophy on safety, and that calls for a fuse only on the supply path.
To decide what is safe one needs to consider failure modes. I guess that it is possible for a failure mode to conflict on where a fuse needs to be, or if a fuse is appropriate means to prevent damage or injury. To address the failure of a ground fault, as an example, there needed to be something other than a fuse to detect that and act on it. Such a device is called a ground fault circuit interrupter, or GFCI. That's getting out of scope though.
The point is to consider the environment, the failure modes that creates, and what to do to protect against them. I gave examples on where one fuse is enough, and where it can take two.