[Unobtanium-inc]

I think the Notchback served several purposes for Porsche.
1. They opened up a new production line with Karmann, which became much more important with the 912 production, so the Notch was probably seen as a trial run of cost and quality with a new builder, Karmann.
2. They were able to add another car to their sales line, without really doing too much. It didn't increase sales very much, especially for the T6 which sold about 700 units total, but it looked good on paper to have another "model" to market and sell.
3. They were able to bleed off cabriolet bodies that may not have been selling as well as the lower end coupes. The cabriolet was a pricey car, so being able to sell them as a cheaper coupe probably worked well for the bottom line.

People normally either love or hate the Notch, they weren't well received when new, but have developed a cult following over the years, as well as many becoming bootleg cabs, though this happens less now because even a Notchback isn't cheap these days.

---Adam