I generally use "services" when there's a glob of logic that works with a number of models and other things. For example, you might have some order checkout logic that has to write to a number of database tables, send email and other notifications, and ping a third party service or two. You don't want to tie all that logic to a single model as it crosses a number of boundaries, and you don't necessarily want it inside a view, as it might be run in some other context such as the command line, or perhaps run from different views.

On the other hand, if there is logic that is neatly encapsulated within a model instance or queryset, I don't see a good reason to artificially make a service for it. Generally refactoring into services is something I do when it's obvious what their responsibilities and boundaries are.