I would define a landrace grain, or any landrace variety as something that has been cultivated in a specific place for a very long time. Over time the people that live and frequent the area where the grain is growing establish relations with the grain and may even select for qualities that are favorable to their pallets or embody traits of cultural and culinary significance. A landrace grain is closely tied to the land and the people who steward it.
When I think of landrace grains, I think about a population of wheat, rice, barley, oat, or other cereal grain which has grown and adapted alongside the changing landscape and people in a particular area. A positive to growing landrace grains is quite simple: that native seed is adapted to that particular environment and requires less inputs to survive and produce.