I also think some do it just for grins or to stretch tongues. I've watched them shoot at hanging droplets a few times then start drinking off of other leaves that hold more water. The bright hanging droplets attract their attention. Chams sometimes shoot at odd things. Watched a jax shoot at a kid's face, another shoot at a cat's ear. I had a male veiled who would roam around on the carpet shooting at bright flecks of mica.