I haven't used DOS much in decades. But during the early days of the PC boom it was everywhere. It basically founded Microsoft and windows. Probably helped the Mac also.

It started out as CPM. Control Program for Micro computers. It was simple and clean. But rapidly grew in power and complexity through multitasking capabilities like DESQView.

Many of the seminal pre-internet communication protocols owe their creation to DOS. Xmodem, Zmodem, sliding windows. I could go on for days. But you get the picture..🧑‍💻