For example, Samsung has their PIA which you can check up on at https://www.samsungadhub.com/pr/ourCapability/smartTVAd.do
Now that alone has to do internet data to supply you with the adverts, checks for firmware updates and more.
As to apps, those can eat data like mad as they call home.
I'd been using my UE48H 3D Smart Tv (2014) for a few months before i decided to connect it to a network to sample it's Smart Tv functions.
Windows 8 64b PC
Huawei mobile wi-fi router (uses a sim card for internet connection)
When i connected the TV to the network (Menu -> Network -> Network Settings) everything seemed fine, but a few weeks later i noticed i was running out of mobile data (charged per MB) in record time and to cut a long story short i tracked the data drain down to the TV. With the TV on i'd watch the metre on the router go up in 5 mb leaps every second or so, as soon as the TV was switched off the drain stopped. At first i though't it was an update of an app that i'd installed, perhaps Plex but as of now i'd estimate it's robbed 4gb of data off me and it still wants more, it surely can't be anything to do with an App can it?
I can't pin-point what i'd done on the TV, if anything, to cause it to rob MB's like this,