Because they are dramatically quieter than piston or gear pumps. Fluid travels axially in closed cavities so delivery is pulseless, and without pressure ripple there is little to excite the pipework and structure. Where noise governs - elevators, theatres, hospitals - that outweighs everything else.

A lift serving occupied floors cannot announce itself on every movement, and the pulseless delivery also gives smoother ride quality than a pulsing pump would. The combination of quietness and smooth flow is exactly what the application needs, and no other pump type delivers both.

Clean oil with adequate viscosity and lubricity, because the idler screws ride on a film of the pumped fluid rather than on bearings. Standard hydraulic oil suits them; water-based fire-resistant fluids generally do not, and using one causes rapid and permanent wear of the idler bores.

No - displacement is fixed by the screw geometry, so flow follows shaft speed directly. Any flow variation must come from a variable-frequency drive on the motor or from a flow control valve, which is a system design consequence rather than a limitation of the pump itself.

Useful pressures for the applications they serve, built up across the several cavities in series, though below what axial and radial piston pumps achieve. A system designed around very high pressure points to a piston pump; screw pumps suit moderate pressure where quietness matters.

Long, on clean oil - there is little to wear provided the fluid film is maintained. Life is governed almost entirely by fluid condition, so filtration and keeping water out of the oil determine service life far more than running hours or pressure do.

Where variable displacement is needed, where pressures exceed the type's range, where the fluid is water-based or contaminated, or where noise simply is not a concern. In those cases a piston pump is more capable and a gear pump is cheaper, and the screw pump's single advantage is not being used.