Moved the coal about in the stokehold to keep the ship in trim and on an even keel.
Equally important, if not more important, ensures that the bunkers close to the stokehold / boiler room are filled with coal so that the firemen can fire the furnaces efficiently. Often also got lumbered (in ships that required it) with clearing the ash out of the way. Normally, coal fired ships had to clean the fires periodically - raking out ash, removing lumps of bat etc. Slows the ship temporarily, but if not done, the fires don't draw properly and the speed falls more slowly but inexorably. Also have to "blow soot" to clear the flues. All these jobs tended to increase the size of the "black gang", many of whom would finish up with a title such as "fireman/trimmer".