I dont get this when you should turn the heating on thing.
I just have the thermostat set at 18c and if it gets colder it comes on. It never get turned off but there again if its over 18c in the house it doesn't come on.
I'm sure we've done lots of reasons to use and not to use thermostats. Reasons I can think of not to use a thermostat include:
Absence of thermostat
Broken thermostat
Temperature is above your setting but you feel cold.
Temperature is below your setting but you don't feel cold.
Wanting heating for a specific reason, eg drying clothes.
Not wanting heating even though it's cold because eg the house is empty or everyone's fast asleep.
Enjoying variation in temperature.