If you pick a doser with its own app (e.g., the Kamoer X1), then you don't need a controller.
I can think of a few things that a controller can add to your setup -
1) monitoring of temperature, salinity, ORP, pH, leak detection, water level, and power usage (if you add automated testing, then water parameters too).
2) the ability to "act" based on the monitored parameters (shut down heaters if overheat, stop ATO if leak, abnormal power usage for anomalies.
3) basic control - turn off return pump / wave maker / ATO / etc for feeding, maintenance, etc. What if I turned the pump off and forgot to turn it back on in 15 min? Things like that.
When I got started, I basically only had 2) to make life easier - if I want to turn something off, I don't want to open the cabinet and unplug stuff, or go through 5 different apps to shut them off. Everything was plugged into a smart power strip and I can open 1 app (whether it's Google Assistant, Alexa, or Siri HomeKit) and push buttons.
As my tank got more mature and added more valuable livestock, I realized that what I had was only "open loop" - meaning there was no automated feedback to tell me if the heaters malfunctioned, or the water parameters out of wack. The ability for the controller to measure something, then act on that information by controlling equipments (or at least alerting me) was a valuable "insurance plan". It's also nice to get everything in one app (instead of checking Inkbird for temperature, Eve for power, etc).
Of course that insurance comes at a cost - $1000-ish for a full fledged Apex setup. Maybe less for Hydros.