By having a dedicated Kuberntes cluster you reduce the blast radius. For instance, if you roll out a new version of your services that unexpected consumes a significant quantity of resources, if you were running that on your production cluster that could interfere with your production workload.
In a similar vein, if you have 10 or even 100 end-to-end test suites, with Krucible you could run them all in parallel, significantly reducing the time taken, without fear of them impacting each other. In your shared cluster scenario you would be limited by the size of your cluster.
