Because the underground tank goes through temperature swings day to night?
Yes, but since the tanks are underground, the temp doesn’t swing that much from night ‘til day, so it’s debatable if it’s worth the effort. I have also heard that water condenses in the underground tank and being lighter than gas, rides on top. That’s okay unless you fill up during or just after the tanker truck fills up the underground tank, stirring up the mixture and sending water to the pump.
Cuz your car and the pipes/hoses close to, and above ground are cooler, and the fuel is more dense.
Be sure you have a few miles to drive, though. In high school, I filled my '66 Nova SS full in the morning. My car sat in the hot sun until noon, and when I got out to it, the fuel was dribbling out of the cap, as it was expanding, and the cap was, well, 1960's vintage, and not a tight sealing one. Left a nice dribble mark in the paint that had to be repainted, no way to fix it. It didn't "buff right out."