Why are there different paddle shapes?

Teardrop Paddle Blades 

Teardrop - where the widest point is at the tip of the blade. This is considered a 'low aspect' paddle with a higher percentage of blade area located at the blade's tip, where the blade is widest. Commonly used for sup surfing and recreational paddling, the teardrop paddle 'bites' early when lowered into the water and is less progressive in feel. The blade will feel 'loaded' early in the stroke and tends to feel encouraging for beginners as they progress to learning sound paddling techniques. 

Mistral low-aspect paddles include the Gold (large), Motu, Kalea, Meherio, 

Rectangular Paddle Blades

A more rectangular blade, with longer straight sides and clearly defined 'shoulders' that angle towards the neck of the blade, puts the max blade width average higher up toward the middle height of the blade. This paddle is said to have a high centre of effort. It's a choice of efficiency, common today with those who race, cruise, and tour; they are favoured for their progressive loading and their promotion of good paddling technique even when fatigued.

Mistral high-aspect paddles include the Keanu

These differing blade shapes, with the same amount of blade area, will perform and feel very differently when compared, so choosing blade area is not a simple matter.