We're talking about smart meters here, I'm aware some people get confused with Nest and Hive which are smart thermostats.
The Gas and Electric companies need to know how much you're using, if you don't go to the box and ring up or go online they will estimate the reading, they also have to periodically check to make sure you're not making it up. Smart meters are attached to this box and give an SMS reading to the Gas and Electricity companies to prevent the faf, the meter still runs as normal. As toolah said the energy companies will know how much to buy from the grid as it is hard and expensive to store.
Smart Thermostats you buy yourself, such as Nest and Hive, although some companies discount them, they connect to the internet and don't turn the heating on when it knows it's a warm day or when your out to save you money.