What's the purpose of the adhesive under iPhone 6s screen? (which doesn't exist in iPhone 6) I work in a workshop that repairs iPhones etc. 

I find it strange that the iPhone 6 doesn't have any glue/adhesive surrounding the enclosure rim under the display.  Yet, the iPhone 6s does.  

When doing repairs to the iPhone 6s without the adhesive, the iPhone passes all tests we've done at the workshop and is perfectly functional.

I have asked a number of Apple technicians about this, and none of them know for certain what the adhesive is for. Some speculate that the presence of this adhesive is just to make it difficult to remove the display, so that non Apple technicians using non-Apple tools will find it harder to remove, while others are guessing it was to make it more waterproof.

Can someone shed some light on this so we can have some more certainty in the workshop when repairing iPhone 6s models? We want to ensure we're not adversely affecting customer iPhones in any way.
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.