No one will publicly admit why Trump uses Twitter. We will discuss the reasons why.
First Reason: Lugenpresse Alternative
Twitter allows Trump to speak directly to the public rather than through the MSM conduits that invariably distort his message in order to undermine his authority.
By establishing the MSM as a "lying press" that never correctly reports on the activities of his administration, Trump further builds the case against private ownership and management of social media. The "lying press" acts as an unelected and privately-governed authority influencing the American public and therefore public policy. To have both a lying press and privately-owned and operated social media platforms means that the American public can not freely get untainted information: thus the need for a public information platform.
Promulgating accurate information proves essential in sustaining the well-informed public necessary to elect competent officials in a functioning Republic. To do otherwise undermines the governing principles of the Republic.
Second Reason: Legally Make Social Media a Utility
By using Twitter to promulgate his messages, Trump builds the precedent and convention for a future anti-Trust action against monopolized social media platforms that often unilaterally influence or even dictate public policy, confounding the efficacy and purpose of the Republic.
Trump establishing this precedent and convention would justify the legal restructuring of social media as regulated utilities rather than privately owned and operated media platforms.
Final Reason: Help EU Do The Same
Evgeny Morozov, a high-powered European intellectual, is the public face of the effort to convert social media platforms in the EU into utilities. The European elite backing Morozov also backs Trump.
Yes, Team Trump is a global phenomenon, with surrogates and allies in every capital.