That subnet had a handful of mainframes that had the same IP address for decades. When we migrated a handful of systems off those mainframes and onto what were then called "minicomputers" (heh!) they ended up in the same subnet.
Those mainframes ran the ERP system for an entire business unit. Every accounting application lived in or talked to a database in that space. Manufacturing information systems that ran assembly lines called into it. The IVR systems talked to systems in that space. Hell, public facing systems called into it (behind a thick layer of indirection :))
Basically, the systems behind that IP space were responsible for ~5 billion in annual revenue, and it was significantly cheaper to lease back those IPs from AWS, do absolutely nothing with them (in AWS), and advertise routes internally for the systems that still remain on those old IPs.