More than likely it's for an option or part that is not needed or offered in the Traverse. This engine is used in other GM cars too, so there may be some item where this is it's mounting point. But, that item is not used on the Traverse. Thinking about the Lambda platform alone, there are some items offered in the Buick version that's not here, like the HUD system. There could be other things too, but I'm just guessing. It could also have been there in the design, but not used later due to a change in the parts that are used. I.E. nothings missing, just different. It's rather normal among all car makers to do things like this.
that part reminds me of the mounting points for engine covers...
for example- on Trailblazers---
the SS V8 and the V8 on the SAAB share a similar engine cover--
The cover is not on the regular V8...
Ive seen post of people ask.. what is this hole here---
eventually someone post a pic of the hole as a mounting point for the engine covers--- they then order that cover for their car--- (they actually looks nice).