From Paddle Surf Warehouse:
There are many reasons why the blade on a stand up paddle is angled. The main reason is with the blade angled forward from the shaft the front part of the stroke is more-effective thus making the stroke longer.
The longer the stroke the more efficient the stroke is. Another reason is that it prevents lifting water at stroke’s end which expends less energy making it easier to go through the water. Bent Shaft paddles deliver more of your effort to the water. You’ll go farther, faster, and with less effort.
A straight-shaft paddle wastes effort lifting water at the end of the stroke; an angled paddle does not.