On a grand piano the struts in the plate add stiffness where they are placed. This can affect the way in which strings interact with each other during tuning. If you are doing a pitch-raise the usual way, the notes you tune just before the treble break are less affected by the subsequent tensioning of strings just past the break. Therefore they can be tuned with less over-pull than if the strut were not there.

By specifying the location of treble struts you can make the over-pull calculations take this strut-stiffening effect into account and avoid having these notes just before the treble break turn out too sharp.

On a vertical piano there may be gaps in the physical layout of the strings that have the same effect on lessening string interaction. If it appears that such gaps have this effect, then their location can be specified as strut locations here as well.