There are three reasons for a buckle at the top of tall boots.
First, the buckle closure allows the leg opening at the top of the shaft to be expanded to accommodate the calf. On most men, the calf circumference 6 - 8“ below the knee is one to two inches wider than the circumference just under the knee (which is the height at which you want tall boots to be). Thus, you can open the boot shaft as you pull boots on over jeans or leathers, then buckle the top to tighten the fit under the knee.
Second, the buckle closure will hold the boots up high on the leg and they won't sag (too far). Boots that sag a lot can cause folds of leather that can rub against the ankle and cause sores. It's best to have boots that are held up by a mechanical method (such as buckles), so they won't sag.
Third, when closed snugly, top-buckled boot shafts will hold jeans, leathers, or breeches inside the boots so when you sit down then stand up, you won't have unsightly bulges of fabric or leather above the knee. That is, it keeps the lines between the fabric or leather and the boots “clean” and have a neater appearance, as shown above. That appearance is desirable by those who wear jeans, leathers, or breeches inside boots. Seldom, if ever, do you see a motorcycle officer with baggy breeches at the knees.
Buckles at the top of boot shafts to hold jeans, leathers or breeches inside them is important if you ride a motorcycle. A motorcycle operator frequently bends and extends his or her knees when stopping, starting, and riding. (And buckles are far better than a laced boot shaft closure, because laces can easily come undone when blown in the wind, and are a pain in the butt to keep re-tying.)