I will second that one, look around, Dumb Question is an understatement that doesn't apply to this thread........
unfortunately it falls on deaf ears, like a neutered dog, they don't get it.....
in any case....something like body oils wouldn't actually set from the temp of the dryer, if not removed in the wash, those oils are more likely to attract dirt from the next wash, giving you the dingy whites....
some stains will set, like chocolate, blood, and dye transfers if not completely removed in the wash
cold washes can be somewhat effective, as I have experimented, but it can take time, as in pretreating stains, and dissolving a detergent and booster in hot water first, then adding...and a series of agitation and soak periods, for several hours...best results came from using a wringer....