According to the Star Trek: Deep Space Nine - Technical Manual the rotation was driven by four factors:
To control the rotation imparted by the processes of docking and undocking spaceships.
To control the additional centrifugal force imparted by transferring ore from the processing centre (at the middle of the station) to docked ships at the outer pylons.
The internal skeletal structure provides only 22 percent, the
remaining 8 percent being derived from the EPS conduit field effect,
which acts as a crude structural integrity field (SIF) system.
While some Starfleet engineers have looked upon this area as
over-designed, it has worked exceedingly well for the Cardassians.
especially in the pylons' ability to damp out lateral and rotational
forces imparted by both docking space vessels and large moving masses
within the pylons.
- To provide thermal control, so that half of the station isn't permanently in darkness and the other half permanently in light (thus preventing one side from overheating).
For example, a continuing topic for debate centers around the reason
ops and the commander's window are aligned on a vector that seems to
relate to no other symmetrical division, 30 degrees away from the ops-
Pylon 3 centerline and 30 degrees away from the ops-Pylon 1
centerline. The most plausible explanation involves the mechanics of
Terok Nor's synchronous orbit about Bajor and the thermal control
rotation of the station.
- There's also the suggestion that the need for a rotational period may be somewhat driven by the desire to have Cardassia Prime in view at all times from the Prefect's office.
A possible correlation exists in which
Cardassia Prime and its parent star would be visible through the
window center at all times.
Ronald D. Moore jokingly suggested that it was because
O'Brien likes the view from his bedroom window to keep changing.