There's more than one reason that humans feel an affinity for trees--not just that they recycle carbon monoxide for us, not just that they vaguely represent humans, with their feet on the ground and their arms in the air; sometimes triumphant humans, sometimes supplicant humans, sometimes staid and dignified humans. They are constructed like humans as well, in that their bark is the tough epidermis protecting their soft innards. Their bark scars where it is wounded; it differs in color from tree to tree; it processes oxygen and water throughout the body of the tree. If too much bark is removed from the tree, it will die.
Unlike human skin, though, each variety of tree has a differently textured bark. Maples and oaks have a thick, deeply ridged bark. Pines have a flaky, flinty bark. Birches, a thin, smooth, papery bark. To some extent it follows that the taller a tree grows, the more texture its bark will have, although that does not appear to be a 100% rule. To some extent it also seems to follow that as a tree ages, its bark gains more character. However, age and height do not necessarily correspond; a tree's ultimate height is determined by its variety, so a shorter, older, tree can have a rougher bark.
Bark serves a purpose for the insect and animal world as well. While it protects the tree's innard from burrowing insects, it also grants insects such as moths and rodents such as squirrels purchase to walk. Gypsy moths lay their eggs in the crevasses created by particularly gnarly oak and maple bark. Deer will eat it, eventually causing the death of the tree if left unchecked.
Humans have relied on tree bark for millenia; as a fire starter, as a plate, as protection from the elements. Like our own skin, we take tree bark for granted, oftentimes stripping birch trees of their bark to decorate our homes. We use bark to dye fabric and leather; to create rubber and spices; to cap wine bottles. We shred it and mound it around our "prettier" plants to protect them from weather. We owe tree bark (and trees) a debt of gratitude.