In the latter they used it to make an inverse warp field, due to an incoming neutronic storm, which was apparently analogous to dropping anchor.
Subspace anchors are a technology that appears inherent in the structure of Star Trek. There exists an asymmetry of (sublight) propulsion: it takes power and time to accelerate, but no power and/or time to decelerate, which shouldn't happen in Newtonian-Einsteinian conditions where both efforts would be mirror images of each other. The ability to drop a drag anchor in subspace, a putative static frame of reference underlying all space, would perfectly explain the asymmetry.
The one thing dubious about "Fair Haven" is that they have to "convert" the warp core to achieve good anchorage. Perhaps adequate anchoring is typically achieved by the subspace components of the impulse engines, and using warp fields for the task is normally considered overkill?