I came across this device just now, and I can't think of any reason for it to have the window that it has.
This isn't just a cutaway display image, either; the datasheet specifically mentions that you should avoid getting glue under it to avoid cracking the glass:
So, what is the purpose of this window? Surely it increases the cost of manufacture, so it wouldn't be there without some good purpose, right?
I can't see it being for trimming, either, as what seems to be exactly the same part is also offered in a different package that doesn't have a window.