What is the purpose of crystals which form on the stems of some houseplants?

When plants ooze liquid droplets out of their leaves that's called guttation.This dries in the form of crystals.The crystals are nothing to worry about. It is sugar crystals. Some plants produce this as waste.It can mean the plant has been overwatered, so the plant tries to get rid of the extra water,then it will dry in the form of sugar crystals. it's not something to worry about.Try just washing the plant and it should come right off.