LB: To answer your question, Presidents like to have high approval ratings because it helps them to advance their agenda. If Congress knows the public is behind the President they may be more likely to pass the President’s preferred legislation, for example. In addition, if the President’s ratings are high, this could prevent his party from losing seats in Congress (or losing control of one or both houses), which often happens at midterm elections, or from losing races at the state level.
As for the specific strategy of having rallies, I don’t know if they’re a good way to increase or maintain public support. But I think the optics of it all may help, since if the public sees him in front of adoring crowds perhaps they will jump on the perceived bandwagon and decide they like him too, despite what they hear from the media.
I actually am not a DT supporter. I’m just trying to answer the question. 🙂