Your phone uses the internet for all sorts of reasons, without asking. Checking for updates, updating your calendar, backing up your contacts etc. It will try to use wifi if it is on and getting a signal, but most phones turn wifi off to preserve battery after some time in standby, so you'll usually still see "using the internet" on your bill when your phone is at home all day.
You can selectively enable the ones you want, but the easiest way is to just turn off mobile data and turn it on when you need it.
I switched mine off when I was nearing my monthly limit and have started keeping it switched off after seeing the effect. It has more than doubled my battery life, and I now only get the samsung wolf-whistle when I look at my phone: As there's no wifi or mobile data, emails sit on the server and are only pulled from the server en-masse when the phone "wakes up" and relinks to the wifi. Otherwise, they're being pushed to the phone individually whenever they are sent.