Plant Secondary Metabolites - why are they so useful to the "Bush Doctor"?

Here's the rub. Why are so many phytochemicals so good for you? Surely it's not a random thing. Most chemicals that are randomly synthesised are not beneficial.

Is it +ve coevolution? Are there rewards to herbivores? Is it because plants synthesise some chemicals for their own benefit; and the eukaryotic human benefits because of his/her basically homologous system? And lastly, is it because any evolved toxic compound is going to be so biochemically "devious" that it will of necessity target a key signalling pathway... and in appropriate dosages would exhibit key pharmacological properties? All of the above?

Lastly, what are the commonest plant secondary metabolites - phytochemicals or botanicals to the bush doctor - and how do the wild critters that eat them cope?

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