I often walk or jog in the evenings for exercise. It's very hot and humid where I live so my exercise clothes go from clean to very sweaty right away. Rather than launder them constantly, or buy more sets, I keep a small bottle of detergent in the bathroom. I run a sink-full of warm water, add a scant cap-full of detergent, and soak the sweaty clothes while I take a shower. Since they're only mildly soapy, it doesn't take long to rinse them out and hang them out to dry (outside if the weather permits, or inside if it doesn't; they usually dry by the next evening, even indoors). It saves water because I'm not using the machine for a tiny load all the time, and it saves wear and tear on the clothes. They're clean enough that I don't mind wearing them all week until the next full laundry day when I can wash them more economically with a full load of clothes.
Of course, this could also be done with at-home or children's play clothes that were a little too dirty to be comfortable but for which you didn't want to do a special load of laundry.