The migration problem
Stone over bare soil sinks: load and water pump fines up and stone down until a "gravel area" becomes a muddy patch with suggestions of stone. The process takes 2–4 seasons and reverses nothing — the fix is rebuilding with fabric. Every old gravel driveway is this story told in ruts.
What fabric does
Geotextile separates the stone from the soil while letting water through — stone stays a stone layer, soil stays soil, and the load spreads across the fabric's tension. It is a $0.15–0.30/sq ft line item that doubles the life of everything above it. Over clay and wet soils it is not optional; over clean sand it is still cheap insurance.
Specs that matter
Woven fabric for driveways (high strength), non-woven for drainage layers. Overlap seams 12"+, pin on slopes, and keep stone thickness adequate — fabric is a separator, not a magic carpet for 1" of stone over swamp. The stone depth rules still apply; fabric just keeps the layers honest.