Prevented Planting is just that - for one reason or another (usually weather, but it gets pretty complicated) federally insured crops aren’t planted in the time frame they were expected to be planted in. This is not Payment in Kind.
1) seen as environmentally bad to plant there (i.e. mess up rain runoff), and
2) to keep from oversupplying the food market with that particular food (ostensibly to keep the food price from going too far down for farmers to make a living), and
3) as you said, as an insurance in case weather or whatever prevented the farmer from planting.