It can be, but how much difference it makes depends on your boiler, the controls you have now and how you use your heating. The main things to weigh up are everyday convenience and how the thermostat communicates with the boiler.
Everyday convenience. Smart thermostats are designed to be adjusted from an app as well as on the wall, so you can change the temperature or the heating schedule without being at home.
Talking to the boiler. With simple on/off control, the boiler heats its water to the same preset temperature every time it runs. Some smart thermostats can use a communication protocol such as OpenTherm to tell a compatible boiler what water temperature to aim for instead. The heating may then run for longer at a lower water temperature, which can be more efficient overall.
Compatibility. Not every boiler supports this kind of communication. What you get depends on the boiler model, the thermostat and how it is set up, so check your boiler with its manufacturer or a heating engineer before choosing.
If you're weighing up new controls, tell us what boiler and controls you have now. There's more on heating controls.