Prosecutors' best move: Charge Trump and seal the indictment - CNN
From Penelope's above citation.........
[A] third reason [for a sealed indictment] could be a factor: when the statute of limitations might expire before the suspect can be arraigned on the indictment.
Mueller and the New York Southern District prosecutors are bound by a Justice Department
policy against indicting a sitting president.
Even if the president does not run for reelection, the five-year statute of limitations might expire on presidential criminal activities that reach back to 2015 by the time he leaves office. That could include planning the "catch and kill' operation used on McDougal.
These timing details are known only by prosecutors and those under investigation.
The issuance and sealing of the indictment would "toll" the statute from expiring. The issuance of the indictment stops the clock, and the sealing may prevent evidence destruction or witness tampering until the case can be tried. A viable prosecution would be possible even if circumstances such as the president's reelection in 2020 prevent a prosecution until 2025.