IMS features are needed for following features:
- LTE call. Advantage: mobile data is available during call (in contrast to 2G call) and it is possible where the 2G/3G network is not available or switched off already
- WIFI call: you can call via WIFI when there is no mobile signal. When you call from any country in the world via WIFI call, it is like if you were in your home country. Conventient when you call family / friends at home from abroad.
When you active IMS features, the phone tries to use modem settings that match to your carrier in your country. When you switch it off, the phone uses a “fall back” modem which works in general with any carrier but then you can only make calls with 2G and LTE and WIFI is only for data. Then, you need a messenger service like Signal for calls via data.