The Unix 'applications' they got for free. The OS itself shipped with a bunch of apps plus the various bits and pieces that now form the Voltron of Xcode and those were all Nextstep apps. Did the Unix part help to market the thing as a workstation? Sure. But, again, this wasn't for love of standards. The actual machine didn't ship with X11. Next didn't, I dunno, join Motif or whatever. The thing was one of the more non-standard workstations around.
X11 didn't even exist when NeXT was founded and its contemporary predecessor wasn't available under the MIT license.
It was often used in the early days of OS X before it had native applications because the X11 version was often better than using Classic, especially after the Intel transition basically killed Classic but not X11.
