1. It means his campaign can get an unfiltered message out to nearly 3 million people instantly, without worrying about what the media will say. In one tweet his words will reach almost as many people as CNN, MSNBC & Fox draw in primetime COMBINED.
2. It means that he is reaching a lot of people who will not normally vote in a GOP primary if he can draw or persuade even a fraction of those people to the polls it will change a lot.
3. As you might have noted from his base speech Trump is not shy about hitting the other people in the GOP race, what do you think will happen if he spends months defining various GOP candidates to millions of people at once?
4. Even if he tweets out something that is complete BS more people will read it than the number of people that will read any person disputing it