Barnes did the same a few times. Essentially, if the ball is at the #8 and the scrum isn't going anywhere (stationary), then if it collapses, the ref will call ball available and not penalise anyone for going down. Also, if the team with the put in wants to win a penalty going forwards, they will need to get both side going forwards, and not just wheel it. So, by shouting stationary, the ref was saying, you are not going to get a penalty, and if you don't use it ...
You will notice different refs also call use it in different ways, rather than being consistent with just those words, 'Use it.' I think I heard Barnes say, at one point (maybe to Dan), 'You have 5 seconds.'