Kitchen rags are used for so many daily tasks -- wiping your hands, drying dishes, cleaning a countertop, etc. -- and most people aren't laundering them nearly often enough. If you cook on the regular, you should be swapping the dish towels out every day or every other day to ensure you're not drying the mixing bowls with a combination of foodborne bacteria, mildew, and ambient kitchen grease.
Here's a not-uncommon line of thinking vis-a-vis bath towels: "I only use it when I'm clean, so it can't be dirty." Alas, that is not so. Sure, you're toweling off after you've washed yourself in the shower or bath, but drying off transfers dead skin, body oil, residue from the products you used, and, of course, water to the towel. The combination of human matter and moisture will aid the development of mildew, leading to that gross smell and resulting in you drying yourself off with bacteria.