Their primary goal is to modernize the tower and make it attractive to future tenants (per their own statements in BPDA filings). The lower floors had smaller windows than the upper floors to begin with. If you look upthread at photos of the tower with the old cladding, you'll notice wider/near-floor-to-ceiling windows at approximately the top ten floors, with much smaller (~waist height) / narrower windows on the other floors. I believe they wanted to have larger windows everywhere, and on the lower floors they had no choice but to rip out the old cladding to achieve this - combine this goal with the above-mentioned goal of preserving occupancy for tenants in the upper floors during the project and one can see where the motivation comes from for this hybrid approach.