Everyone else commenting is missing the point, because as you've said, Prometheus can discover targets via k8s endpoints/pods dynamically already, and you can run a sidecar to reload the config file.
The main point of Prometheus operator is to federate access to Prometheus configuration so teams can manage configuration that the built-in Kubernetes service discovery doesn't let you control from an application side. Things like scrape interval, recording rules and alerts, etc. ServiceMonitors, PrometheusRule CRDs basically let app owners ship these aspects of monitoring with their applications, instead of having to have a hand-off between the service owners and the SRE teams managing the monitoring infrastructure.
The main point of Prometheus operator is to federate access to Prometheus configuration so teams can manage configuration that the built-in Kubernetes service discovery doesn't let you control from an application side. Things like scrape interval, recording rules and alerts, etc. ServiceMonitors, PrometheusRule CRDs basically let app owners ship these aspects of monitoring with their applications, instead of having to have a hand-off between the service owners and the SRE teams managing the monitoring infrastructure.