If Fort Sumter had remained in Union hands, it would have been useful as an observation post reporting on Confederate port activity, especially blockade runners. As it was, holding Fort Sumter caused the new Confederate government to manifest its intentions as other than peaceful.
Okay, I do not comprehend this statement. It doesn't make any sense.
We are postulating a peaceful coexistence between the North and the South, so why would there be a blockade for which Ft. Sumter would be useful?
Your premise is that Ft. Sumter was useful to the Union in a war with the South, but this particular topic thread which we are currently discussing is in response to the question "what good is Ft. Sumter to the North?"
Under peaceful circumstances it was utterly worthless to them, and if you are going to have to postulate a war to make it useful to them you are pretty much acknowledging that there never was any intent by the North to leave the South in peace, that the plan always was to have a war with them.
As it was, holding Fort Sumter caused the new Confederate government to manifest its intentions as other than peaceful.
Major Anderson's forces committed what was regarded as the first hostilities in the war. They burned the guns at Ft. Moultrie. The people of Charleston had been led to believe that the Forts would all be turned over to them, but by burning the guns it became clear that the people of Charleston were being treated as a hostile enemy.