A class has a constructor so that when an instance of the
class is created, the fields of the class can be setup to a
initial valid state.

Abstract classes define partial implementation of a public
contract. Which means that it may implement some of the
methods and contains partially implemented methods that are
marked abstract.

Since abstract classes can have partial implementation and
such partial implementation can include fields of the class,
a constructor becomes necessary so that those fields are
initialized to a valid default state when they are created
thru the constructor of their concrete subclasses.