You want wheat (and other grains) to be dry before harvest. Harvesting when wet increases losses due mold growth. Wet stems cause sticking and blocking in the harvest machinery. In many countries it's common to apply a defoliant: You kill or half-kill the foliage of the crop plants, and wait a small number of days, so grains and stems get drier before the harvest.
This use of glyphosate has nothing to do GMOs (there is no GMO wheat in markets anywhere). In a small number of countries, it's forbidden to use chemicals (plant poisons) to force desiccation.
I dislike this being spelled out in terms of "increasing yield" as it is actually more about managing quality. Pre-harvest applications of glyphosate are mostly a thing of the past in wheat anyway, the varieties dry down better now - it was mostly a artifact of the Canadian transition from drying crops in windrows to leaving them stand. The genetics have caught up now.
There are no varieties of Wheat, Oats, or Barley registered in Canada that a resistant to glyphosate.