A presidential term is incredibly short, and there are many things a president hopes to accomplish. And even if he knows that the investigation is going to prove him right, the process is deeply distracting, and gives political opponents extra energy to resist him.
You could say the Mueller investigation was an Obstruction of the Presidency itself, since it impeded Trump from performing his job to the fullest.
He knew there was no collusion over a year ago. He did this to obstruct Trump and help the Dems with impeachment if they were suicidal enough to move forward with it.