Because chips fall away from the cutting zone instead of into the fixture. That removes recutting, which damages finish and shortens tool life, and it stops heat being carried back into the part by accumulated chips. In heavy cast iron and steel work the difference is substantial, and it is what makes long unattended running practical on a horizontal machine where a vertical would need intervention.

Spindle time. On a single-table machine the spindle stops while the operator unloads, reloads and sets the next part. A pallet changer moves that work outside the machine, so loading happens on one pallet while cutting continues on the other. Over a shift the difference in spindle utilisation is large, and it is usually the main financial justification for the higher capital cost.

Four, using the rotary B axis to present each vertical face of the part to the spindle, plus whatever a right-angle head or fifth axis adds. Combined with a tombstone fixture holding several parts, that means a complete four-sided component can be produced in one operation, removing the setups and the accumulated positional error that the same part would incur on a three-axis vertical machine.

A tall, usually square or rectangular fixture block mounted on the pallet, with parts held on each of its faces. Rotating the pallet presents each face to the spindle in turn, so one program can machine many parts without the machine stopping. Tombstone fixturing is what converts the horizontal configuration into high output, and designing it well is a large part of getting value from the machine.

For short runs, one-off work and prototyping, where the setup and fixturing effort is not recovered across enough parts, and where a vertical machine's quick, visible setup wins. Also where parts are large and flat rather than prismatic, or where floor space and capital are constrained. The horizontal is a production machine and its advantages are realised through repetition.

A queue of pallets, loaded in advance, that the machine draws from automatically. It extends the pallet changer principle from two pallets to many, allowing a machine to work through a night or a weekend on prepared jobs without an operator. It requires reliable tooling, in-process probing and disciplined preparation, and it is where the highest spindle utilisation in a machine shop is usually found.

Capital cost, floor space and complexity. Fixturing is more expensive and takes longer to design, programming is more involved with multiple faces and a rotary axis, and setup errors are more costly because more parts are affected. Operator visibility into the cut is poorer. Shops generally add a horizontal once volume on a family of parts justifies it rather than as a first machine.