Why would the Union army need to resupply Fort Sumter?

There are at
least two reasons for which you can say the Union needed to resupply Fort Sumter.

On one level, you can say the Union needed to resupply the fort because the fort was in
Southern territory.  The fort was in the harbor of Charleston, South Carolina.  That meant that
it could not get any supplies on its own since the South felt it was an enemy outpost.  It was
running out of supplies and had less than a months worth of provisions.  Therefore, it needed to
be resupplied from the North.

On another level, however, we can talk about
why the North felt it needed to keep Fort Sumter.  The major reason for this is that it felt to
many Northerners like a point of pride.  They did not want to feel as if they were giving in to
the South.  If they had simply let the South chase them out of Fort Sumter, they would have felt
weak.  For this reason, they felt that they needed to keep the fort.  If they were going to keep
the fort, they had to (as discussed in the previous paragraph) resupply
it.