Kerrith (or Fowey, as it is called outside our novel) is significant to Rebecca because it's the nearest major town to Manderley, and its inhabitants are closely tied to the estate and the de Winters. It serves as a connection between the isolated forests of Manderley and the outside world. The town is around 3 miles from the estate and is a center for local commerce and infrastructure (page 135: for location).
This location illustrates the isolation of Manderley and how it draws a dividing line between the de Winters and the people of Kerrith. When a ship crashes near the cliffs, the narrator joins the citizens of Kerrith to observe the boats in the water. She wishes to lose her own identity and join them (page 288), which underscores the isolation of Manderley and the extent to which Rebecca's ghost has haunted her to the point of consuming her every thought, decision, and conversation with Maxim. Kerrith represents the life and status she left behind when she married de Winter.
Mapping this location on COVE reveals how the narrator is constrained and viewed differently or viewed as a higher-class woman (even though she is not). Kerrith represents the ordinary, everyday community where the de Winters conduct business (alongside Frank), and sharply contrasts with the Gothic atmosphere of Manderley.
picture source: Home | Fowey Harbour