The steam chest (or steamchest) is the "reservoir" for collection of steam as it passes between the superheater header and the inlet port to the cylinder.
The advantage of a large steam chest (as is the advantage of any reservoir) is that fluctions in pressure as the steam passes from the steamchest into the cylinder are reduced. The higher the steamchest pressure, the greater the quantity of steam that can be delivered to the cylinder while the inlet port is open, and the higher the cylinder pressure at the point of cut-off. Maximizing cylinder pressure at the point of cut-off serves to maximize the work done by the steam on the piston. In alternative words, it serves to maximize the area within the indicator diagram.
Ideally, the steamchest volume should equal (or exceed) the cylinder volume, but never came near this in FGS locomotives. One of the modifications that Wardale made in developing The Red Devil was enlargement of the steam chests which are easily visible on the photo below. In fact, the extent of the enlargement was limited by other constraints such that their volume increased only from 33.2% of cylinder volume to just 35.5% compared to an ideal minimum of 100%. By contrast, the 5AT steam chest volume is almost exactly 100% - see line  of FDC 6.