The signoz helm chart is just to install the signoz application, and the k8s-infra chart is to instrument a cluster with otel monitoring that reports back to the signoz application. So yes you'd install both if you want to run signoz in the same cluster you are monitoring.
My guess for why they are split is to support a topology where you have a dedicated monitoring cluster, and other clusters where your workloads are deployed. In this case you'd deploy the signoz chart to the first cluster and k8s-infra chart to the others.