Dirty or greasy stains cannot be cleared by washing with water and we need detergents. Why?

Yes dirty or greasy stains cannot be cleared by washing with water and we need detergents for the same because water cannot find contact with the greasy or oily molecules, and hence, it cannot wet the surface.

We use detergents whose molecules are hairpin like in shape. When we dissolve detergents in water the molecules of detergents get attracted to water and when clothes are dipped in such water the hairpin molecules get attached to the grease, and hence, water and greasy dirt interface is formed. Therefore, detergents are used to reduce the surface tension between water and greasy stain. When clothes are rinsed in water the greasy dirt is washed away by running water.