When a fixture relocates to a new position, the plumbing has to follow it there. The new position is set during the design phase, and the plumbing work that follows is the skilled work of running new pipe and connections to that new location. What makes plumbing reconfiguration work very different from installing plumbing in a new open building is what inevitably gets left behind: the pipe runs and connections that no longer serve any purpose at the old fixture location. Those abandoned runs have to be properly capped and made safe rather than left abandoned inside walls and floors, still pressurized and live, waiting to eventually leak in a place where you cannot see it or access it.
Joining new pipe to existing old pipe of a different material or from a different era is a technical detail that has a clear right way to do it and several wrong ways. Copper to PEX plastic requires a specific transition coupling. Galvanized steel pipe to copper requires isolation sleeves and the proper fittings to prevent galvanic corrosion. Old cast iron to new plastic requires transition fittings designed for that specific combination. The junction has to be pressure-tight and reliable, and the materials have to be compatible and play well together over time without degrading, corrosion, or failure.
Working inside a floor or ceiling that is only partly open and exposed is a significant physical constraint on the plumber. You cannot run pipe the way you would in new construction or open framing because existing structural members, joists, electrical runs, ducts and other utilities are all in the way competing for the same space. Some direct paths are impossible to achieve. Others require removing more floor or ceiling area than was originally planned to provide access and workspace. The surfaces that had to be opened and removed have to be restored afterward to match the original appearance and finish. The opening itself is planned to be kept to the absolute minimum size that allows the work to be safely completed, and the surface restoration is planned to occur in the locations where it is simplest and least visible to do.