@Fiona
For me the most important feature is that it can share locations between Apple and Android devices. The free version is a bit limited but you can save locations and get notifications when a someone arrives/leaves that location. This is quite useful for me so that I know my daughter has arrived home from school., or if she is at a friends house that I don't approve off.You can also look at the history of where the device has been, so I can see if she went by chick-fil-a or went straight to school in the morning.
There is an emergency message feature that sends a message to those in your circle... Yeah, you can do some these things with other built in features of the phone, but this is just one quick and convenient way to track and communicate with your family.