Referred to as a secondary nutrient behind nitrogen, phosphorous and potassium, calcium is finally starting to take its place in the ranks of important plant nutrients. It is true that NPK is used in greater percentages than calcium, but calcium is used more by weight and volume than any other nutrient. Instead the focus on calcium has been more as a soil buffer to help adjust pH. Calcium is of huge importance to both the plant and the soil in many more ways than simply moving the pH scale. It plays a major role in the physiology of the plant, strengthening its physical structure and helping in protection from disease attack. In the soil, the importance of calcium is many fold, including the reduction of soil compaction and helping to provide a better environment for the proliferation of beneficial microbes.
Some research even suggests that calcium plays a role in decreasing weed populations.
The more calcium that is available to that cell, the stronger those walls become. If calcium is limited the walls are as weak as jelly. As more calcium becomes available, those walls take on the strength of cinder blocks. The stronger the cell, the stronger the plant, and the quicker its recovery from the enormous pressures that it's faced with in the garden. This works for both leaves and roots.