I believe the top head tube extension is, as others have said, a way to get the handlebar higher without resorting to a sloping top tube. Lots of older riders (like me) don't have the flexibility for a big saddle to bar drop, but like traditional geometry with a horizontal top tube and drop bars.

I don't understand why there's a big extension on the bottom lug, though. On a frame designed for 700c wheels, the standard fork axle-to-crown measurement is limited to a small range based on the headset stack height, tire/fender clearance, and typically a ~13mm head tube extension below the intersection of the bottom of the down tube and the head tube. The longer lower extension would profoundly affect the handling unless adjustments in the frame geometry were made to keep the trail in an acceptable range.

Here's an Ahearne frame with a similar top extension (but standard lower extension). It also has a fairly long fork steerer with headset spacers and a tall quill stem to get the handlebar in the stratosphere.