Four reasons, one whimsical and three practical:

Whimsical: During my research I came across the fact that you can order bullet-resistant glass in custom sizes (if you have the money), Since the other three sides of the house will be proof against multiple hits from .50 caliber rounds, I thought it pleasingly symetrical to give the multi-paned southern window-wall bullet-resistant panes.

Practical 1: As I'm single and expect to remain so for the foreseeable future, I will need only the third level of this house for my own use. I thus intend to rent out the two lower floors as efficiency apartments monthly/weekly. It occurred to me that if these efficiencies were built like small fortresses (instead of the barely-converted overage motel rooms with cheap doors and shoddy windows as most here are), this would be a valuable selling/retention point, particularly for single women living alone.

Practical 2: I found a government engineering study from the UK on soundproofing which discussed how multiple panes of slightly-different thickness set slightly *off* parallel with one another - so as to damp-down/break-up sound wave patterns - were one the best ways to insulate windows against sound transmission. The study also suggested smaller window sizes, but since I'm designing for solar-gain winter heating that's not an option. But given that bullet-resistant glass is considerably denser than normal glass, it would somewhat balance out the necessary larger window size as regards soundproofing.

Practical 3: My mother makes most of her income these days from monthly/weekly rentals. The biggest *expenses* she encounters come when renters move out, in the form of recarpeting floors, repairing punctured drywall and replacing broken windows. Poured concrete surfaces coated with diamond-hard epoxy resin solve the first two issues; fixed window panes crafted from bullet-resistant glass take care of the third.