Before C# 7.2 we had protected internal modifier. This really means protected OR internal, that is – member A is accessible to child classes and also to any class in the current assembly, even if that class is not child of class A (so restriction implied by “protected” is relaxed).
private protected really means protected AND internal. That is – member is accessible only to child classes which are in the same assembly, but not to child classes which are outside assembly (so restriction implied by “protected” is narrowed – becomes even more restrictive). That is useful if you build hierarchy of classes in your assembly and do not want any child classes from other assemblies to access certain parts of that hierarchy.
We can take example that Jon Skeet provided in comments. Suppose you have class
public class MyClass {
}
And you want to be able to inherit from it only in current assembly, but do not want to allow to instantiate this class directly except from within this class hierarchy.
Inheriting only within the current assembly may be achieved with internal constructor
public class MyClass {
internal MyClass() {
}
}
Preventing direct instantiation except withing current class hierarchy may be achieved with protected constructor:
public class MyClass {
protected MyClass() {
}
}
And to get both – you need private protected constructor:
public class MyClass {
private protected MyClass() {
}
}