As my dad (pathologist) put it, "Nature is a lazy bitch. She'd rather repurpose the same thing 50 times than develop something novel."

So add up random biochemical pathway * X million years of evoltion, and one begins to understand the fun of drug discovery and testing.

"We know this is the pathway involved in heart disease. We know this chemical compound is effective against that pathway.

... Now why are all our mouse models getting erections?"