I see this "debate" as a combination of things:
First, and most important to Mr. Trump, this is an opportunity for publicity for him personally. His actions for decades have made it clear that the most important person in the world of Donald Trump is Donald Trump, with everyone else jockeying for a distant second place.
Second, it is very likely he views this "debate" as a chance to incite Tea Party loyalists and others who (mistakenly, in my view) think he is suitable as a Presidential candidate to see him talk about issues of Presidential importance, and, potentially, ask him to throw his hat in the ring, likely as a third candidate. He would do so not because he wants to be President but because (circling back to #1), it would be a further, prolonged opportunity for publicity.
Third, in light of the considerations mentioned in #1 and #2, this is an opportunity to attempt to make the GOP candidates look foolish, the hopes that this will make him look better as a potential candidate himself. He will do this by asking slanted, emotionally-charged questions (e.g., "What are we going to do about those Chinese bastards manipulating their currency?") that will get the audience spooled up and make the candidates look milquetoast, since they can't use the same kind of outrageous tone that the Donald will employ.
Fourth, it gives him an opportunity to declare a winner, so that, if his pick wins the nomination, he can take credit for it (which brings us back to #1).
Donald Trump is, in my view, a distasteful man who will not provide a forum for intelligent debate. (This is the same man who was calling the Chinese "motherfuckers" during a political fundraiser during his brief tenure as a candidate for President.) He will very likely stage a "debate" which reflects poorly on everyone involved, including him. My own view is that participating in the debate is a mistake for anyone who is serious about running for President.