Political Qestions of the Day

If President Trump is not a "subject" or "target" of Special Counsel, Robert Mueller's Russia/Trump campaign "collusion" investigation, as asserted by Deputy Attorney General, Rod Rosenstein, why would he have to sit down for an interview with him? In what capacity/role would Trump be expected to answer questions, if he's neither? I've heard no one provide an answer to this...nor, have I even heard anyone ask these questions.

This makes no sense. The whole thing is bewildering...so, I'm left wondering what Mr. Mueller has on Mr. Trump...