"Here" to me means stop what you doing NOW and return to my side. Here is one of the first commands that should mastered. Becuase with one word you not only stop the dogs forward momentum but you return him to safety. Some examples, your well mannered pooch ends up nose to nose with an aggressive dog looking for a fight. Here brings him back Whoa leaves him standing nose to nose until you get there. Same thing is true if your dog is the trouble maker. If your absent minded pooch wanders into the street, whoa stops him in his tracks waiting for the next bus. In the field whoa may be more usefull in steadying your dog as well as keeping them safe but even there it has the potential to leave them standing in harms way.
Just my opinion.