Usually it's a benefit that comes from having something want to eat the plant. Fruits and seeds that are attractive to eat can often result in wider seed dispersal, for example. But there are plants that benefit from having rooting animals grubbing in the ground near them, because of fertilization or better soil aeration or better water penetration/retention in the soil. That alone can be enough reason for the plant to store food energy.
It certainly seems to have that effect on me. Of course I don't have to dig them up. It's curious to me that other legumes did not select the higher fat content as well.
Thanks for your insight. I attended a lecture and am reading a book by Ropert Sapolsky of Stanford who is very bullish on the idea that all life is driven by the desire to reproduce its genetic material. I think that is why I've been focusing on these evolutionary arguments. It seems almost to be lower on Maslow's pyramid than survival in many ways. Sometimes it seems like what we are actually doing here is trying to offset the unnatural habits that our contrived environment has tricked us into adopting. I used to think of peanut butter as being on the healthy end of the spectrum. But I am certain now that it undermines my ability to fulfil those physiological needs.
Thanks again for your response.