Originally designed as a standard Deep Space Starbase, it was converted into a military base with the primary purpose of serving as a first line of defense against the Borg. Deep Space 5 was constantly upgraded with the latest in sensor technology to scan for approaching Borg ships. (Last Unicorn RPG module: Star Trek: Deep Space Nine Core Game Book)
In 2373, DS5 became the temporary home for thousands of Thallonian refugees after the fall of the Thallonian Empire. The station was packed to capacity, with even the conference rooms serving as shelter.
Also in 2373, Deep Space 5 reported the complete destruction of the colony on Ivor Prime. It was subsequently determined that the Borg were responsible, and were launching a new invasion of the Federation. Admiral Hayes sent this information to Captain Jean-Luc Picard on the USS Enterprise-E. (TNG movie: First Contact)