Most often it is when I want to tantalize my husband. Suppose we are going out someplace where I would wear a dress (a play, a concert, a cocktail party, etc.) and as we are are on our way from the car to the building I might say "Oh dear, I was in such a rush getting dressed that I forgot to put panties on. (If nobody is in sight I might even lift my hem to show him.) Or, just before we get out of the car, I might slip my panties off and leave them in the car. A couple of times we have been on friends front porch about to knock on the door to attend a party and I slip down my panties and ask him to put them in his pocket. Oh, and I have a few times (we've been together for a long time, our kids are in college now) been with him in a pub or restaurant and have gone to the ladies room and when I get back to our table I have opened my purse or briefcase to show him my panties or even all undergarments (actually these days, I am more likely to have a backpack with a laptop in it than a purse and I haven't carried a briefcase in years).
All the time, dont wear panties.