Heavy Duty Coil Springs

[chevyc1068]

What are heavy duty coil springs? Do they add to the amount of weight you can carry or do they just stiffen the ride, or both. Because I'm looking at upping the GVW and posibly adding an inch or two for convenience of bigger tires. Would heavy duty springs help that matter at all. Like in adds or in catalogs there will be standard spring and then heavy duty ones. So whats the difference???