Why should cables be strain relieved?

Cables that are in motion are compressed and stretched.
This is called relative motion. This relative motion must be braced to prevent the cables and inserted connectors, contacts and associated peripherals from being under constant stress. If a tensile force passes through the cable, e.g. directly on a contact, the connections could be damaged.
The consequences include, inter alia, loose contacts, signal interference, heat generation or electric arcs, which could develop into a fire or failure of the components.