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.

What area are in blue

Not true it depends on the person