The Rema (Yoreh Deah 198:48) states that a woman emerging from the mikveh should take care to be met by a friend. This is meant to avoid a situation in which her first encounter out of the mikveh is with an impure creature, such as a dog or a pig. Some sources raise a concern that such an encounter could have a negative spiritual impact on a child conceived following the immersion, and would advise the woman to immerse again if this happened.
Therefore, by greeting the woman as she emerges from the water (there are opinions that she should shake her hand or make some type of physical contact), the balanit ensures that the woman's first encounter is with a pious person who will be a positive spiritual influence. (This is true even if the balanit is niddah at the time; the Rema is referring to a different, inherent type of impurity.)
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