We have a Catalpa tree growing on the riverbank in our front yard. We enjoy the Catalpa's giant, heart-shaped leaves and, at times, the leaves' nectar "springs" make for great conversation.
Even the bees love Catalpa leaves... at least this time of year.
The Catalpa's nectaries do a good job of attracting a variety of insects. The plan is to provide an attractant for the wasps, bees, etc... and in return, gain some protection against herbivores. In other words, providing "refreshments" for all these predators increases the chances of them finding caterpillars, or other pests on the Catalpa leaves... and with all these hunters milling around, it's more likely the pests will be killed or parasitized.
Yellowjackets aren't the only visitors to the Catalpa's nectaries. I've seen plenty of other kinds of visitors... like this iridescent bee.
Sometimes, our backyard entertainment is our Catalpa tree.