Java having an explicit ”interface” construct is one thing I didn’t like about it, because it muddles the notion of a class implicitly having an interface (a notion that clearly exists in C and C++, by way of header files if nothing else) with that construct, while on the other hand there is no a-priori reason to have a distinction between Java’s interfaces and pure abstract classes. Both specify an interface to be implemented. And Java 8+ muddles its concept further by allowing default methods and static members.
The important thing is to distinguish between interface and implementation, and that is relevant to any class, whether it implements a separately defined interface or not.
Java (following Objective-C) does need a differentiation between interface and pure abstract class - this is because it is single inheritance - a class can have any one class to inherit from but it can have many interfaces.
That doesn’t follow. The restriction could have been defined in terms of allowing at most one parent class to be non-pure-abstract. And there are lesser restrictions hat would have been conceivable as well. Java is single-inheritance only in the sense of the particular interface–class distinction it makes. For example, since Java 8 you can inherit method implementations from multiple interfaces.
The important thing is to distinguish between interface and implementation, and that is relevant to any class, whether it implements a separately defined interface or not.