Good grief! These things are dangerous! Why would anybody plant these on purpose?
Jewelry; mostly. Because they are so colorful and shiny, they have always been (and still are) very popular for making beads (hence the "rosary" part of the common name) though this is a very, very bad idea. The seed coat on a mature RP is actually quite hard and impermeable, so much so that, even though one bean contains enough abrin to kill many, many elephants if you were to swallow a mature one, you probably would be OK since it would in all likelihood pass through your system unbreached. But if you drill holes in them....... Nonetheless they are still used as beads with great frequency. There is a jewelry seller who often shows up at the local rock and mineral show who specializes in stuff made in Southeast Asia and her stock contains a lot of necklaces with drilled RP seeds, along with other somewhat less toxic (but still very poisonous) seeds like Coral beans (
Erythrina sp.) Oxen's red eye (
Mucuna bennetti) and the seeds of the ladybug tree (
Ormosia sp.). And there is a conductor who is sometimes on the train I take into the city on Wed.'s who wears a cuff of the things (someday, I'm going to have to get up the courage to actually tell him he probably shouldn't, if I see him)
The seeds have some other uses. They are very, very consistent in both size and weight, so in a lot of parts of Asia (most notably India) they are the standard method of weighing out units of gold, similar to how ancient jewelers used to measure the weight of gems in carob seeds (from which we get the word "carat"). Crushed they make a kind of glue that is used to hold pieces of gold together for melting while fabricating (again not something
I'd do) On the darker side, I have heard that they used to be a pretty popular murder weapon there as well (in the days before poison detection methods were really all that good, or even extant). Abrin is soluble in water and when the water evaporates, it tends to crystalize in needle shapes crystals. According to what I have read, these needles were a popular method of murder since being stuck with one hurts about as much as an ant bite (regular, not fire ant) and the symptoms of abrin poisoning are supposed to look a lot like heart failure.
Even the plant has some interest, as it supposedly closes down when the pressures drop before a storm (hence another name for it, barometer vine)
As far as I know, seed is still perfectly legal to buy and sell (at least I was when I bought the sample I use for reference, but that
was almost 20 years ago) at least here (I think I may have heard something about it being restricted in some European, though I think the restriction was more along the lines of not selling jewelry made from the seeds without disclosing the dangers, the same way there is now a warning on the packet if you buy castor beans.)