Those extra-floral nectaries are one of the the main reasons that I grow so many yardlong beans. They attract wasps & ladybugs in large numbers. Both of those are voracious insect predators, that help to control aphids & caterpillars in my garden. Some who order ladybugs for their garden find that they just fly away; having yardlongs or cowpeas in bloom will provide a food source for the adults, and they will stick around.
Then too, I just enjoy eating yardlong beans.
Ants are attracted too, but to me they are just a nuisance. The wasps on & around the plants are unusually docile, as if they get drunk from the nectar; they will ignore my presence as I harvest (unless I grab one accidentally). The ants, on the other hand, are highly aggressive, and will attack anything which touches the plants... probably the reason that the plant developed those nectaries. The black ants, though, sometimes bring their aphid "cows" with them... usually the ladybugs will keep the aphids down to manageable numbers.