As the rain water is the purest form of water, this can be used in lab due to:
Absence of ions
The purest source of natural water is Rainwater. The rainwater directly comes from the condensation of water in the presence of the sun. The water evaporates from the lakes, rivers and seas. All the impurities are removed during the vaporization process, and then waterfalls directly into the earth in the form of rain. Rainwater can be used in labs because of the absence of ions in it.