These LED flashlights are powered by two or more D batteries. The advantages of D battery flashlights is the common and inexpensive battery type. In addition, the large form factor and weight often doubles as heavy blunt emergency backup weapon for night watchmen and the like. D batteries have large capacities which means they will last longer than AA and especially AAA battery flashlights.
The disadvantage is that despite their large size, D batteries are still only 1.5 volts, so these lights aren't typically nearly as bright as 3 volt lithium CR123A batteries. They also can't boast about having a 10 year shelf life, like lithiums can.
D battery flashlights for decades have been the "standard" but they are extremely rare in the high-power flashlight world due to their size, weight and low voltage limitations.