Leaf shape relates to its purpose?

A leaf shape is crucial to the plants survival. Plants have broad leaves to absorb sunlight in order for photosynthesis. However, a great surface area leaf to a desert plant would be disadvantageous due to the risk of loosing precious water. Therefore, most cacti reduce its leaves to spines and photosynthesis occur in the green stem. As far as uses suited to its shape, that doesn't seem to be a correlation since evolution favors the traits that enables the plant to survive not its use to humans as medicine or craft.

Leaves occur in nature and their shapes are determined by natural selection and evolution through thousands of years to make them more adaptable to the slowly changing environment. Humans use different kinds of leaves in various ways dependent upon each leaf's shape, chemical composition, strength, versatility, etc. It is not the shape of the leaf that relates it to the purpose, but rather it is the purpose that seeks which leaf should best be used (from among the millions around).