All new XC40s come with a Vodafone 100GB, 12 month SIM card installed.
Whilst Volvo On Call uses it's own separate eSIM built into the car, the "personal SIM" as is it called gives you access to some of the apps (like Google Local, Spotify etc) and things like live traffic and over the air map updates.
Of course if you use stuff like CarPlay or even just stream music over Bluetooth from your phone, then of course the data for the phone is used.
If you are a modern parent who has adopted the "parenting by iPad" model then you can use the pSIM to create a wifi hotspot in the car so your little ones can connect their tablets in the car and stream Peppa Pig if that's your thing.
If you don't intend to do streaming but like the idea of the connected services, then as mentioned, both Three and Vodafone do pre loaded, 24 month SIMS with 24GB of data, no contract and they are both just plug in and go. Choosing one over the other is really based on coverage in your area and aren't expensive. If you do manage to use all the data up, throw away and buy another
Links here for reference
Three 24G
https://amzn.to/30F4cuT
Vodafone 24G
https://amzn.to/2P0i4xJ