It gives the most capacity and the most lift for its footprint, because the ram works directly upward in line with the body and almost no height is wasted on mechanism. Wherever the clearance beneath the load allows one, it is the efficient choice.

Collapsed height. The whole body must fit beneath the load before lifting begins, and on a low vehicle or a machine close to the floor there is simply not room. That is precisely the gap that toe jacks and low-profile jacks exist to fill.

Not reliably. The pump pickup depends on gravity to stay submerged in the reservoir, so a jack laid over will pump erratically or lose prime entirely. Some designs tolerate limited tilt, but the safe assumption is upright operation on firm level ground.

Reach without additional collapsed height - the saddle screws up to meet the load before hydraulic lifting starts. It recovers some of the clearance problem and is useful where the gap beneath the load is large but the jack still has to fit in initially.

On higher-capacity jacks, where the number of hand strokes needed to reach full lift becomes genuinely tiring and slow. Shop air does the pumping instead. It only makes sense where compressed air is available at the point of use, which usually means a fixed workshop.

With the jack unloaded and the ram fully retracted, open the release valve and operate the pump several times to purge trapped air, then close and top up the oil to the correct level. Air is the usual cause of a jack that feels spongy or will not reach full height after transport.

Firm and level. A bottle jack concentrates the whole load into a small base, so soft ground, tarmac in hot weather or a cracked slab can let it sink or tip. A spreader plate under the base distributes the load and is worth using anywhere outside a solid workshop floor.