As already mentined - it's hygiene reasons.
I can go most of the week in one pair of jeans (I wear a different pair of jeans for work, so I suppose really it's 2 pairs of jeans) ...... I change my grundies every day ...... I couldn't imagine what the insides of my jeans would smell like if I wore them the same length of time with no underwear !!!!
I'd love to hear from people who do go commando - what about the hygiene ??? I don't go care how clean people are, things sweat during the day. I'm quite a clean person (I shower EVERY day, sometimes twice, changing my underwear at least once a day) and I wouldn't dream of wearing my grundies for more than one day !!!