Because an uncorrected position is accurate to metres, which is useless for construction. Corrections remove atmospheric and orbital errors, bringing accuracy to centimetre level. They come either from a network service over a mobile connection, or from your own base station broadcasting to the rover by radio.

Network RTK receives corrections from a national or commercial reference network over a mobile connection, so no base station is needed - simpler, but dependent on network and mobile coverage. Base-and-rover generates its own corrections locally over radio, working anywhere including sites with no coverage, at the cost of setting up and securing a base.

Typically centimetre-level horizontally with RTK corrections under good sky conditions, and noticeably worse vertically - commonly around twice the horizontal figure. The vertical limitation is regularly underestimated, and is the main reason level work is often done with a total station or level instead.

Indoors, under any structure, in tunnels, and anywhere the sky is substantially obstructed - deep excavations, dense tree canopy, tight urban streets between tall buildings. Reflective surfaces also cause multipath, which is more dangerous than no signal because it returns a plausible but wrong position.

More satellites visible at any moment, which improves both availability and reliability in partly obstructed sites. Where a single-constellation receiver loses its fix beside a building or under trees, a multi-constellation receiver often maintains one, which translates directly into working time rather than waiting.

An inertial system that lets the receiver compute the ground point even when the pole is not held plumb, removing the need to level a bubble at every point. It speeds up measurement significantly and lets points be taken in awkward positions, such as against a wall or into a corner.

Yes. GNSS gives coordinates in a global reference frame, and they must be related to the project's own grid and datum through a site calibration or transformation established from known control points. Skipping that step produces internally consistent results that do not match the design.