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.
So yes -> you need an operator to dynamically change config of prometheus. If not -> all your changes wont matter because they will be not reflected in prom.
If all of your pods have the same scrape settings, there's not much benefit to the operator. But if each pod or set of pods needs custom scrape settings - say different scrape intervals or custom tag re-writes, the operator will let you define each set of scrape configs separately as kubernetes resources. You can store the definitions alongside the resources they scrape instead of having a single large complex config in your prometheus deployement. This would be especially beneficial if you have multiple projects owned by multiple teams, all scraped by the same Prometheus.
Granted I also don't use the operator, but I've looked into it. With complex enough deployments it would simplify things.
The same with annotations approach - its very limited.
The two features I need (and can get without the provider) are: 1. If I (manually, explicitly) change the configmap, prometheus can pick it up without being restarted. This is provided by the side-car container. 2. If new pods come online they are automatically detected and scraped by Prometheus. As long as they have the annotations saying: a) this pod should be scraped, b) scrape on this port, and c) scrape at this URL - then they will be scraped.