I am just speculating now , but if Trump does go third-party, it would most likely mean that the Democratic candidate (most likely HRC) will be elected but - here’s the catch - without a majority of the popular vote.

That means that the GOP and TP would be able to claim that HRC is not the legitimate President (the fact that this was also the case with GWB in 2000 is irrelevant) and use that as their excuse to ignore of stonewall everything she tries to accomplish.