Form an "Hourglass" Bead
Gaps move. They constantly grow wider or narrower as the temperature and humidity change. A shrinking gap is the lesser problem; it just compresses the caulk. But when a gap widens, it puts a major strain on the seal. The caulk has to stretch without tearing in half or pulling away from the surrounding surface.
To prevent those failures, you want a bead that stretches easily in the middle and sticks stubbornly along the edges. An hourglass-shape bead accomplishes both of those goals. It stretches easily because it’s thin in the middle and sticks well because it has wide “adhesion zones” along the edges. Backer rod makes a perfect hourglass bead easy. In situations where you’re not using backer rod, a perfect bead may be easy, difficult or impossible. But use it whenever you can—you’ll get a seal that will last much longer.