Current and heat
Copper cross-section sets resistive loss; the same copper spreads heat away from hot components. Both improve with weight, but so does the etching difficulty.
Process implications
Thicker foil needs longer etching and plating, tighter process control and often a wider minimum trace. The design rules must be agreed before layout, not after.
Harsh environments
Thermal cycling and vibration stress plated barrels. Heavy copper improves current capacity but also increases the copper balance challenge, so stackups are built symmetric where possible.
