Weed or not to weed

Each garden I have ever had has a few hidden treasures that tell me much of what went before.  At the base of an old brick wall I found the pretty yellow  toadflax.  Now why would anyone call it toadflax without a reason.  So has anyone got an answer?  I am looking it up in my old herbals to see if there is anything to explain its name.but can't find anything.

Black Medick (above) is another of those little plants that delight me when I found it in my old garden.  It was growing between the paving slabs and when I brushed it all these black seeds popped out.  I had to take it with me when I moved it was such a pretty thing to have growing so low down as to be inconspicuous.    Medick must mean medicine - so what was it used for?