Floor space, principally. It sits in the pipe run with suction and discharge on the same axis, occupying roughly its own base area and needing no separate foundation block. In a plant room or a packaged booster set, that footprint advantage usually outweighs the access disadvantages.

Motor and pump share a rigid mounting with no separate baseplate or coupling to align in the field. That removes shaft misalignment, which is one of the most common causes of early bearing and seal failure on horizontal pumps, and it makes installation substantially quicker.

A mechanical seal supplied as a pre-assembled unit that fits as one piece, so it can be changed without dismantling the pump or setting the seal faces by hand. It removes the setting errors that cause repeat seal failures and shortens the job considerably, particularly on a pump installed in a pipe run.

Enough overhead to lift the motor off, which is easily overlooked in a low plant room and is worth checking at design stage rather than at first service. Some designs allow the seal to be changed without motor removal, which reduces the requirement considerably.

Generally not. NPSH tolerance is limited and they are normally installed with a flooded suction under positive pressure. Where a lift is unavoidable, a self-priming design or a vertical turbine pump is the appropriate choice rather than trying to make an inline pump work.

Because the common duties - potable water, treated water, condensate and process fluids - either require it for hygiene or would be contaminated by cast iron. Stainless also suits the thin sections that stacked stage construction uses, and it resists the corrosion that would otherwise change stage clearances.

By adding stages, which makes the pump taller rather than wider. That is the format's central advantage: additional head costs height, not floor area. The practical ceiling is the room's headroom and the clearance needed for maintenance above the motor.