How do I know if I need a humidistat?

Purpose of a dehumidistat

How do I know if I need a humidistat?

You have a conflict here. They are two different animals.

Make up your mind. Dehumidification removes moisture from the air Humidifying adds moisture to the air.

In a home system you can humidify using water and all sorts of delivery methods. Usually people humidify during the winter months if the humidity is low.

Air conditioning usually dehumidifies as a side effect although dehumidifiers exist.

You don't have the ability to run heat and AC at the same time in a home system, but you can in some cars. You see the effect of dehumidification, by running the AC and the heater at the same time when it's raining and cold. You will feel much warmer.

The temperature and humidity range that your comfortable at in winter and summer help make that decision. You can do more dehumidifying in AC mode with some tricks. Sacrificing temperature for humidity or running the fan on low speed rather than high.