Power-to-weight in a handheld tool, with no exhaust and no ignition source. A hydraulic saw sustains cutting power a petrol saw matches only with more noise, weight and fumes, and that battery tools cannot approach - and it does so in confined and wet conditions where neither alternative works.

Yes, and it is one of the main reasons to choose them. There is no exhaust, so trenches, tunnels, tanks and building interiors are all workable where a petrol engine would produce a fatal atmosphere. The power pack stays outside with only the hoses running in.

They remove the electrical ignition source, which is why they are used in refineries, tank farms and mines. Cutting still produces sparks from the work itself, so the hazard assessment covers the operation as a whole rather than being satisfied by the tool alone.

Yes - hydraulic tools run wet or fully submerged without difficulty, which is why utility gangs and rescue crews use them in flooded excavations. Fully subsea use additionally calls for corrosion-resistant construction and usually a biodegradable fluid. Fresh-water rinsing after immersion matters as much as the build.

Hose length and the pressure drop along it. A long hose run reduces the power reaching the tool, so pack position and hose sizing have to be planned. This tether is the format's main practical disadvantage against a battery tool.

One matching the tool's flow and pressure requirement, which is a specification to check rather than assume - a pack that supplies one tool adequately may starve another. Portable petrol or diesel packs suit field work; electric packs suit indoor and enclosed use.

Rescue services, utility and pipeline crews, tunnelling and confined-space contractors, and industry working in flammable atmospheres. The common requirement is serious cutting power somewhere that a petrol engine cannot run and a battery tool would not be enough. In each case the alternative is either unusable or not powerful enough.