It seems that in Drupal code anything that is neither hook nor Plugin nor Controller nor any other thing with a standard functionality should be a Service.
For sure, services make sense when we want to give other modules the possibility to change the functionality that we are writing.
But I can think of a lot of scenarios where it is absolutely sufficient to have a normal class. When we just want to create an instance, call a method, nothing else.
How can I decide if a class is sufficient, or if a service makes sense?
Edit This is not opinion based. There must be a clear and objective reason why we use services in Drupal, and I want to know this reason.