"Marginal" lands might have low quality soil. Or they might be on a hillside that rises sharply. Or they might be in an area that gets limited rainfall. In any case, they are the last to be planted under good conditions and the first to be avoided under bad conditions.
But there are uses for marginal lands. Most often they are used as grasslands for cattle, sheep or goats. A farmer might use native grasses or non-native seed. Either way, it is important to establish good ground cover to avoid the loss of soil through erosion.