A useful man or houseman was a male domestic servant that took care of many of the daily needs of caring for a large house. He worked behind the scenes, and would rarely be seen by family or guests. He would rank below a footman but above a hall boy or house boy; he would perhaps broadly match the rank of the housemaid, and would often assist in the same duties, particularly when they required a strong arm (e.g., running the mangle, heavy duty floor scrubbing).
While his exact duties might vary greatly from period to period and household to household, the useful man does what is useful; polishing boots, cleaning windows, clearing the front walk, maintaining fires (perhaps particularly being responsible for carrying loads of coal and firewood), running errands, and doing small repairs. He would be a helper available to the butler in whatever capacity was needed, with an emphasis on odd jobs and those that needed a strong back.
IN14