My guess is the precast is to anchor the bottom half of the tower back to the heritage facades.
Having glass shoot up from a 3 storey limestone facade would look disproportioned.
Based on the rendering, the beige precast will stop soon, and the fins that are in the recess facing Yonge will widen out and go up the tower for a bit.
That looks like it'll provide a transition from the weighty heritage facade,
then half density beige precast sections,
then lighter fins (which look thinner as-built than in the rendering),
then all glass to the top (at least on the Yonge side without balconies).
I think that all is to provide visual interest or blend into the Yonge streetscape.