Why are rov vehicles better than manned vehicles?

The obvious benefit to using ROVs rather than manned vehicles is first and foremost the cost. It costs a great deal more to build a vehicle large enough for a person, then you have to pay that person, who must be very skilled, to pilot the vehicle underwater. This is a much greater level of risk, therefore a greater deal of money. ROVs constitute a much reduced level of risk and cost. You can build the vehicle smaller, and the operator controls the vehicle from a relatively safe location(usually from the offshore platform, but also from onshore). The risk if anything goes wrong is just lost equipment. Although this can be quite significant, it is nothing compared to losing a human life.