Yellowjackets are lousy pollinators. From a gardening point of view they are pests. They scavenge and steal and eat whatever they find and sometimes destroy. In this particular case, I have caught one munching on the stem. The wasp has bitten into it and probably munched on the sap. It definitively was not just cooling off its heels in the shade, her mandibles were moving and she took her time on that spot.
What the ones on the leaves were doing I have no idea, but I never saw one even near the blossoms.