In official Arc, there's not much of a point. As akkartik says, tagged types are used to designate macros, but that's about the only thing that cares about them.
The slight benefit is that 'annotate is the single most obvious way to do type labels in Arc, meaning people don't usually have to invent their own representation for dynamically typed values. As we explore future directions of the Arc language, we can compare type ideas 'annotate, and in many cases we can implement them using 'annotate too, in order to try them out right away, in a way that's often compatible with other people's code.
In other words, 'annotate only has the value we Arc users give it, but it's there, so we occasionally do add value to it.
