A joist is used:

A joist serves as a critical horizontal support system within a building structure. It is primarily used to support floors and ceilings, making it a fundamental component in constructing stable, safe living or working environments. Joists are typically installed parallel to each other and work collectively to distribute loads across walls or beams, ensuring that any weight placed on the floor or ceiling is adequately supported.

While joists can indeed support floors, their role extends beyond that singular function. They provide horizontal support across various parts of buildings, including the areas beneath roofs and floors. This versatility is essential for maintaining the integrity of the structure.

Other options tend to misrepresent the specific applications of joists. For instance, while they contribute to holding up a floor, saying they are exclusively utilized for that purpose overlooks their broader use in structural support systems. Similarly, limiting joists to roof support or specifying them just for sheetrock support does not capture their primary role in structural engineering. Joists are more accurately described as horizontal supports that can be used in various applications throughout a building.