Design principle of triangular rudder planform

Recently saw a Cape North 43 on the hard (a design from Ted Brewer) with an interesting rudder planform. More designs from that era seem to have a rudder with a chord widening going further down, however mostly not this extend:

https://sailboatdata.com/storage/images/sailboat/drawing/cape_north_43_drawing.jpg

Does anyone know the design principles of planforms like this? What are the advantages compared to more rectangular designs and what insights led to stop designing planforms like that?

 

It might make more sense to consider the rudder and the skeg it's attached to as a single unit with a quadrilateral planform, with the rudder itself acting like a very large flap.