As as my understanding use Window Service should come in place when some automatic process should be done on the basis of Scheduling.
Why should we use Web Service or WCF because already have Data Access/Business Logic Layer in our application. Also I think, for scheduling purposes Window Service is the good option.
Your comments
Because by exposing this functionality in a reusable service, other applications/platforms might take advantage of it. Of course if your DAL/BL is only used within a single application, you get no benefit from moving it into a separate service.