buildprop | Android build.prop , default.prop editor | Frontend Framework library

The way I like to think of services in Angular is that they are the place where to put any kind of business logic, from complex async stuff to simple sync transformations of data.

Why put the logic in a service? One good reason is that this make the code much simpler to test and forces you to keep in Components only what is strictly related to visualization and managing user interactions.

Having said that, there is no reason to think that services must return Observables. They must return what makes sense to return: Observables in case of async logic, anything else appropriate in other sync cases.

Here some thinking around your specific case.

First of all I question why to use mergeMap? You should probably be able to achieve the same result using map, which is an operator that performs a transformation on the data emitted by the source Observable. In other words I expect this logic, which uses map and does not require buildProp to return an Observable, to work.