A lot of "engineers" nowadays don't know the concept of per-user/customers configs and how to build/expose them to non-technical staff.
The main appeal of feature flags is simplicity and being a low-hanging way to apply per-customer/user configuration, few platforms allow true a/b testing (amplitude comes to mind but I'm sure there are more).
Feature flag can (usually does??) have another meaning, as a technical feature rollout mechanism, so you can roll back quickly without a deployment. A way to manage risk on teams that make hundreds of commits/deploys a week. You can then often send a certain % of traffic through the feature or not to look for early warnings.
I don't like feature flags for config. Just build a config system into the product UI - a settings page basically! That might have some bits you configure as the site owner, and some that are self-service.
Statsig and launchdarkly are the one that come to mind for me.