Well, you have a phone, and a subscription "is almost as good as free"! I would be chomping at the bit to get that connection, install everything, and get it all working.
You're the boss over its use, and you can for instance disable mobile data use, so that you only use data when connected to WiFi hotspots.
BTW, a socalled 'unlimited mobile data plan' is not unlimited at all. It is governed by a "fair use policy" cap. But if you don't use streaming video, watch live TV, or game a lot, that limited "unlimited data plan" is enough for most users. But exceeding it can get expensive.