Why not should we movie common functionality in non abstract

If you need a default behaviour of those functionalities then, you can create a non abstract class with default implementation but mark those members as virtual in that class, and the subclasses can define the funcationlity if required.

But if there is a requirement that every subclass should define it's own implementation, then there are two create an interface or create an abstract class and mark those method abstract which will enforce the inheriting classes to provide the functionality/implementation for those members of the paretbase class.

Hope that helps!