Why do we need services?
Plain Ruby methods solve most of the same problems as services most of the time. When a task can be done with just a method, that is almost always the preferable choice.
Services are not competitors or replacements for methods. They are an addition to them, another option in the same toolbox as primitives, methods, and classes.
For example, removing duplicates from an array can be done with an array, a hash, or a set, depending on the situation. The choice between a plain method and a service works the same way: use whichever brings an advantage in that specific case.