Why is my stem shim angled?
I came across this article that somewhat explains why it is angled.

Basically what it does is provide a slight angle without having to use a stem that is adjustable.

What it fails to mention is that you can't place spacer rings above the stem meaning your handlebars must be at the top of steerer tube. Too add to all of that nonsense you have to use a top cap that has a hole that is off center to accommodate the angle.

It seems really stupid to me but if those extra few degrees of angle are what you need to dial in your fit then I suppose having an angled shim would be useful.