The Avatar of Death (as summoned by the skull card in a deck of many things) has both truesight and darkvision to a range of 60ft. The darkvision seems redundant, because truesight already lets you see through all darkness as if it were normal lighting. Is there any benefit to having both abilities?
The Avatar of Death probably not, a PC possibly
The Avatar of Death would likely always use truesight, as darkvision has less benefits.
If a PC was granted truesight and darkvision of 60 feet a PC would likely always use truesight up to the 60 foot range. A mechanical benefit to having darkvision would be the possible ability to increase the range of the darkvision beyond 60 feet. Using class features such as the Ranger Gloom Stalker’s Umbral Sight, the range for darkvision would be extended to 90 feet.