No, you can totally use the default UE4 ACharacter or your own derived character for your blueprint.
There is no particular purpose in the case of the tutorial. There is no functionality in it. It is just what I always do in every project with just about every default class. If the base class is in C++ I can add C++ functionality to it later. However, if my blueprint only inherits from a default class then I can’t go back and add C++ or convert blueprint functionality to C++ easily when the time comes.
Also when I make a C++ project, it automatically makes these derived classes for me.