EmberJS: when not to use computed properties in services?

The EmberJS guides explain how computed properties work. They basically listen on changes and then a cascade wherever they're dependent on is recomputed.

I was wondering, can you create a service without computed properties? Are such services handy to have at any time?

I simply find it hard to understand when something should be and shouldn't be a computed property (other than internal variables that you simply use inside the function of a service, that should just be a normal variable).