Buoyancy is usually required by the ROV to offset the in water weight of the whole ROV, the preferred method of operating an ROV is to have the ROV in water weight as near to neutral as possible. Some people prefer a slightly heavy ROV and others prefer a Slight positive Buoyancy.
The ROV manoeuvres and can use it's available propulsion power better if the ROV is neutral or close to neutral, the mass the Propulsion system has to move is only the static load and not an additional dynamic load imposed on the ROV by the Equipment fitted to the ROV.
Operational and many times personal requirements decides what mode the ROV will operate in, as a rule of thumb the smaller the ROV the less choice is you have in preference. The Buoyancy also more or less determines how much real Payload an ROV can carry.

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