Research suggests that we can form stable impressions of other people based on as little as a single behavior. what is the ultimate benefit and importance of this ability?

The ultimate benefit and importance would be Presentation of Self

Presentation of self-refers to a situation where your presence exclude both verbal and nonverbal messages that tell others about your characters and credentials. This will make you appear more trustworthy and dependable as a person.

Getting a good impression of a person's character by a single small behavior is beneficial as a guideline to determine whether you can trust someone or not. If that behavior makes you uncomfortable then most certainly you want to be careful how you proceed with this individual.