In addition to the service discovery, few other things are available when you use Istio on top of Kubernetes:

• Blue green deployments with request routing to new release based on percentage value given by user.
• Rate limiting
• Integration with grafana and prometheus for monitoring
• Service graph visualisation with Kiali
• Circuit breaking
• Tracing and metrics without having to instrument your applications
• You will be able to secure your connections at mesh level via mTLS

You can read more about the advantages of having Istio in your cluster here.