Grant was having a serious problem with the lenght of his supply line and the amount of coal and provisions needed to keep his army and the naval ships around Vicksburg. Confederate Cavalry had captured two or three steamboats and coal barges like the "Blue Wing" on the Arkansas side of the Mississippi in November and December of 1862 and were becoming quite a problem for the thinly escorted supply convoy boats.
Besides the primary purpose of securing the Lower Arkansas River corn crop necessary for the Confederates in Arkansas because of the drought that the state had suffered under in 1862, Arkansas Post was necessary to attempt to create a safe haven/harbor for confederate blockade runners to cross the Mississippi river. The Ponchartrain was being refitted with railroad rails at Little Rock as an Ironclad Ram and had it been able to move from Little Rock to Arkansas Post it could have severely interdicted those Union supply shipments.
The problem was that the Ponchartrain require 10 feet of water to navigate in because her boilers and engines were below the main deck. However, the Arkansas River between Pine Bluff and Arkansas Post was shallow and was usually only 4 to 6 feet deep most of the year. Too shallow for even the heavy Union city class ironclads which require at least 6 feet of water. This deep water is the reason that I believe that Arkansas Post was built where it was instead of upriver a few more miles where the big Federal Ironclads would have a harder time attacking the installation.
At the time of the Arkansas Post expedition there were 5 of the Ponchartrains heavy guns available for refitting. Three at Arkansas Post in Fort Hindman and two at St. Charles, which besides those at Arkansas Post, those two guns were captured by the Union navy on flatcars at Deval's Bluff ready to be shipped back to Little Rock.
This is what I believe that the Confederates were up to at Arkansas Post, and what promted the Union higher command to comment that they had to attack Arkansas Post when they did before the Post became "Too Strong". The Fortifications themselves were finished in January 1863. So what else was left to be done to make these fortifications "Stronger" from a Union viewpoint?
Combine the Ponchartrain with the fortifications at Arkansas Post and Parsons Cavalry and the Post's Infantry patroling the west banks of the Mississippi looking for targets of oppertunity from the supply convoys and you would have a serious threat to Grant's Vicksburg operations.