We recently stumbled on a 2010 article from Backpacker.com which explains the benefits of Merino wool better than we ever could. There are so many amazing, inherent qualities, the author was able to write a full 2-page article on it. In a nutshell:
Merino is soft. Traditional wool is not. "Different breeds of sheep produce wool fibers of varying thickness. The finer the fiber, the softer the wool." Merino is 15 to 24 microns wide. (Compare that to human hair, which is about 80 microns.)
Natural climate control. You know Merino works awesome in the cold, trapping heat so you stay warm; but did you also know it releases heat when it's warm outside, allowing your body to stay cool? "Wool acts like a thermos, keeping you warm on chilly days and cool on scorchers."
Synthetics vs. Wool. Yeah, you'll pay more with the latter, but here's why. While synthetics "tend to be cheaper and lighter, and dry faster... merino fights odor, and feels warmer and less clammy when damp.... Wool is also natural, renewable, and biodegradable and can be produced using less petroleum and fewer toxic chemicals."
Odor control. "Since the wool fiber’s core absorbs water and its hydrophobic exterior repels it, there’s no moist surface for microbes to grow on, making wool naturally antibacterial. So you can wear it for days without developing the arresting bouquet that can plague synthetics."