There is no doubt that certain plant substances interact with our bodies in a manner which enhances our health. There are many examples of such interaction including bioactives such as salicylates from willows, resveratrol from grapes, EGCG from green tea, and curcumin from tumeric. The xenohormesis hypothesis provides a potential explanation for why such beneficial interactions occur. The general idea is that xenohormetic molecules are synthesized in plants in reaction to adversity (lack of water, predation, or temperature extremes). Animals react to these signals in a way which increases health and this ability has been maintained and selected for as a result of natural selection. The term xenohormesis is derived from xenos (which means stranger) and hormesis (which refers to health benefits resulting from mild biological stress). The idea being that the stress experienced by the plants is transfered through bioatives to animals that eat them. At this point the xenohormesis hypothesis appears to be much more viable than the antioxidant hypothesis, although it's always possible that both have some viability.
[Xenohormesis: Sensing the Chemical Cues of Other Species, Konrad Howitz and David Sinclair, Click here for article at cell.com]