What's the purpose of the adhesive under iPhone 6s screen? (which doesn't exist in iPhone 6)
The iPhone 6s and iPhone 6s Plus were actually designed to be more water resistant (although not officially so). I've seen inside these and the difference compared to earlier models is obvious (the adhesive you refer to is one example). 

You can read more about this here.

However, do not mistaken this to mean the iPhone 6s has any sort of official IP rating. It does not. You may also be interested in my answer to this question.