If you want to look clean and smell nice, your clothes need to be washed frequently. Most clothing is made of tiny threads that are woven together. As you go about your day, dirt and odors get trapped in the weave of your clothes and can only be removed by washing.
Clothes must be jiggled and swished around quite a bit in water—as is done in a washing machine—to best remove dirt and odors. Detergent is added to the water to help the process: it can break up oily particles into smaller pieces that can be whisked away, and it can surround other dirt particles and pull them away from fabric.