Mainly to remove visible soils and – only to a small extent – to reduce microbial colonisation of the skin (e.g. in case of contamination with spores of C. difficile).
Handwashing is far less indicated than generally believed. Hence, it should be an exception in clinical practice. Hands should be washed before each shift, after each shift, and after using the toilet.
In addition to its limited efficacy – compared to hand disinfection – frequent handwashing may lead to dry skin, damage the skin's barrier function, or even trigger irritant contact dermatitis.