The original purpose was they were meant to be space station uniforms, and indeed throughout DS9's first four seasons they did adhere to this somewhat faithfully in that whenever we see someone from a visiting ship or with a ground assignment (like Cal Hudson or the people at Starfleet Command), they were wearing the TNG uniforms. The matter became muddled because of Generations and their original planned new uniforms. The plan there was that those uniforms would have replaced the TNG uniforms as the "ship and ground assignment" uniform. They would have been worn by Voyager's crew, they'd be worn by the crews of visiting starships on DS9 and Sisko would wear that when he went to Earth in Homefront, as would the personnel at Starfleet Command.
But because Generations abandoned the new uniforms at the literal last minute, they didn't have enough time to come up with another new uniform design, but they still wanted the movie to have a different look from TV series TNG, so their only option was to use the DS9 uniforms. Even then, they only had enough time to make tailor-made uniforms for Patrick Stewart and Brent Spiner. Everyone else had to wear the DS9 cast's uniforms, and indeed Jonathan Frakes work Avery Brooks' uniform and LeVar Burton wore Colm Meaney's. Michael Dorn, Marina Sirtis and Gates McFadden had to stick with the TNG uniforms because at the time there were no DS9 uniforms to fit them. Voyager went with the DS9 uniforms because it was felt the cast looked better in them than the TNG ones, and because the DS9 uniforms looked "more modern" than the TNG ones.