Not only do they put atmospheric niitrogen in the soil via small nodules on their roots other plants can take advantage of that
The Mexican Indians grow them as part of a trinity ... maize for the runner bean to climb up and melons to grow in the shade of the maize the bean provide enough nitrogen to the soil for the other two , the melons slow the moisture evaporation .
Sadly it seems that this idea is not very successful here in the UK. where we don't get alot of outside melon growing heat and plenty of sun .
I tried this with sweetcorn, climbing French beans and squashes, in the polytunnel. The beans wound themselves around the cobs
and stopped them getting fertilised or developing properly, then became too heavy for the corn stalks to support, so I had to put canes in anyway. The corn showered the squash leaves with thick pollen which turned black. The only thing which cropped properly was the squashes
. It was almost impossible to get into the patch because it was so crowded, so weeds grew including a bruckmansia (sp?) (thorn apple) which appeared from nowhere, stank the place out, is enormous and highly toxic. It was difficult to get in to water as well.
So now I grow each plant the traditional (for GB) way, each with their own area.
pgkevet, no I've not tried hard with the overwintered bean roots because they're never in the right place (rotation) and there's never a full row. I suppose though it would help.