By opening an ever-wider hole in an enclosure, one can smoothly transform from a fully shielded enclosure, to a PCB with ground plane. This continuously worsens the EMI performance of the system (radiation passes through the hole, proportional to its size, roughly speaking, and give or take positioning of components/traces with respect to it and other nearby ground), but remains better than a random hairball of grounds and signal wires, for example.
A key insight from such a thought experiment, is the finding that external noise currents are largely carried around the periphery of the board. Consider the board exposed to a magnetic field for example; when the board is oriented perpendicular to the field lines, it acts to block them, inducing an equal and opposite current around the perimeter. The board itself serves as shield, and therefore any cable shields must be terminated into it, preferably around the edges, and with very low impedance (multiple wide ground connections).
Tim