Good question. We see someone putting gravel in a hole and assume they know there is some good reason to do so. After all, we have gravel roads and gravel stuff is put under buildings so it must be good, right? That gravel is typically crushed limestone, graded, and has enough fine particles to bind the whole pad/road into a big block. No reason for anything like that at telescope piers. As for drainage, gravel around the pier provides a path for water to run to the bottom of the pier. If the gravel allows water to drain around the pier, where does the water eventually go? There is no drain so it ponds in the gravel at the bottom and eventually soaks into the ground which causes the soil to expand which tends to move the pier. Then it dries out and moves again, gets wet and moves again, etc. Will it make a difference in your images or will you have to re-align every season? In some soils, definitely. No way to know for sure everywhere, but why use gravel, or sand, when it is working against what we want to accomplish?
If you pull a foundation that has been set on gravel for a while, the air gaps in the gravel have been filled in with dirt. That dirt came from somewhere which means things have been moving a little for the soil to displace the air voids. Will it be a measurable movement? Certainly not if we are talking about fence posts but, we are talking about arc-secs. Again, why would we do things that work against us?
For loose fill around a pier in the pier hole, think about a simple lever. Force times distance. Consider a pier, 6' in the ground and 3' above the ground. If the in-ground part is poured to fill the hole, it is restrained by the undisturbed earth and we have a 3' lever arm from the ground to the top of the pier. If we over excavate the hole and form the underground part of the pier and then fill in the annulus with sand/gravel/dirt, The sand or gravel is a fluid as far as structure is concerned so the pier is not restrained by the earth and the lever arm is now 9' instead of 3' and the base of the pier is a hinge. It's not quite that simple but you get the idea.
Indeed I do. Thanks for the response and for putting it in layman's terms. There will be no gravel below my pier.