Clients sometimes ask for a process by name because that is what the last shop used. The better question is what the joint needs: the base metal, the thickness, the position, whether it is indoors or in the wind, and what the inspection requirements are. The process falls out of those answers.
Gas tungsten arc welding (GTAW, or TIG) is the choice when the root pass has to be perfect and the inside surface matters: stainless and alloy pipe, thin-wall tubing, and any joint where heat input must stay low. It is slow, and that is the price of control.
Shielded metal arc welding (SMAW, or stick) still wins in the field. It runs off an engine-driven rig, tolerates wind and less-than-perfect fit-up, and welds in every position. Heavy carbon steel pipe and structural repairs on a job site are its natural home.
Gas metal arc welding (GMAW, or MIG) and flux cored arc welding (FCAW) are the high-deposition shop processes. MIG gives clean, fast fill on carbon steel, stainless, and aluminum indoors. Flux cored carries more deposition on heavy structural and, in its self-shielded form, works outdoors where a gas shield would blow away.
Submerged arc welding (SAW) is for thick sections and long seams: girth welds on large pipe turned on rollers, longitudinal plate seams, and heavy build-up. Deep penetration and very high deposition, with almost no spatter, but it needs a flat or horizontal joint and the setup to feed flux.
Most code pipe and heavy structural joints end up as a combination: a TIG or stick root for penetration and a clean ID, then MIG, flux cored, or SAW for fill and cap. A shop with all five processes and welders qualified across them can build the joint the right way instead of the only way it knows.