Most schematics treat ground as a single node. Every PCB layout tool will happily let you pour one. But copper is not a wire. It's a plane, and current through a plane takes the path of least impedance, not least resistance. At DC those are the same thing. At 50 MHz they are not.
Return current follows the signal
High-frequency return current hugs the trace directly above it, mirrored in the plane below. Cut a slot in the ground plane under a fast signal and the return path has to detour around it, and now you've built an accidental loop antenna. This is the single most common self-inflicted EMI problem we see on hand-routed boards.
The fix isn't exotic. Keep the plane continuous under anything that switches fast. If you must cross a split, route the signal through a stitching via near the crossing so the return current has a short path back.
One plane, three jobs
On a mixed-signal board, the plane is simultaneously a low-impedance reference for your digital lines, a shield between layers, and a thermal spreader pulling heat off a regulator. Optimizing for one can quietly hurt another: a thermal relief pattern that isolates a pad for solderability also raises the local impedance right where you don't want it.
None of this means overthink every board. A two-layer breakout with a slow I2C sensor doesn't need this level of care. But the moment you're routing anything above a few megahertz, or running enough current to care about heat, the ground plane stops being an afterthought and starts being part of the schematic.