Does form follow function? Or does function follow form? This is a question a lot of designers, engineers, and artists debate. Is it more important for something to be aesthetically pleasing, or for it to be supremely functional?
I think that we see a wide range of answers to this question in the underwear world. You have the posing straps, the barely-there items, or the new singlets with the seat cut out - all of which are about aesthetics and not function. (Alright, you might argue about the functionality of the ass-less singlet, but I'm not going there.) On the other hand, you have extremely utilitarian options, like your traditional tighty-whities, or - to take it one step further - the "union suit". Yes, I do own one, and yes, it is very warm and comfy when you have to work outside on frigid winter mornings in the Northland. And then there's the matter of full-cut boxer shorts, which are neither aesthetically pleasing nor functional, at least in my opinion.
Anyhow, right now I'm in my second pair of undies for the day. The first pair - grey cotton 2(x)ist trunks - was more toward the functional side, since I spent the morning driving and doing dirty work. Yes, I'm a country person, and yes, I do dirty jobs sometimes. When I'm doing such work, I choose undies that are comfortable and practical, and generally inexpensive, in case they get wrecked with animal byproduct. And when I'm driving long distances, regardless of whether I'm going to be doing hard work, I choose comfortable pairs. Because really, who wants to sit for hours and hours with a thong chafing in your crack while you drive?
The second pair, black CK hip briefs, is more aesthetically pleasing. It's not the most fancy pair I own, but it's one of my better everyday pairs. If I may say so, I clean up pretty good, and I like my undies to reflect that. Nice clothes should include nice undies.
If you were to look in my drawer, you would find a number of pairs in there for specific occasions/purposes. I have a pair of UnderArmour boxerjocks for race-day running. I have a gym-ready jockstrap from when I used to frequent the gym. I have a few pairs of cheaper undies for when I'm doing outside/dirty work. And I have a few pairs of "special occasion" undies (let the reader understand).
So... do you have specific pairs with specific duties? Are there times when you would wear one pair and definitely not others?