The point of the feature flag tool is to control feature flags. They are not just for A/B testing and slow rollouts, and most products are not on the scale of Amazon, where you need to have a feedback loop for the rollout. Of course there is always going to be some overlap between tools, where many analytics platform allow you to do the rudimentary version of this.

The main point of a FF system is that you have a runtime-controllable mapping from user attributes to feature flags, where a set of logic rules can dictate who is exposed to what feature. With most analytics-tool based rollouts, you would be able to only directly set an attribute and you then hard-wire that attribute to the feature toggle in app code, which gives you a lot less flexibility and always requires a engineer in the hot path when it comes to doing rollouts.