Motivations to implement ztunnel generally came from two areas.

First, and most importantly, it serves as a means to implement the real goal: waypoints.
For various reasons outside the scope of this document, there is a desire to move from a sidecar based architecture to a "remote proxy" architecture.
However, this has one glaring issue: how do we get the traffic to the remote proxies, while maintaining the zero-trust properties that Istio is built upon?