You are right, a thermostat is more expense. Maybe OK for one hive, not OK for many hives.
I have few bees and only want to use heat and thermostat on one hive near home. I already have the thermostat.
Inside hive temperature goes up and down with the weather for hives with no heat.
Inside hive temperature goes up and down with the weather for hives with heaters and no thermostat.
If weather goes colder, with no sun, and much wind, then a hive with heater and no thermostat, inside temperature will go down. Bees may like the heat from fixed terrarium heater, but inside temperature will go down.
If weather goes warm, sunny, and no wind, then a hive with heater and no thermostat, inside temperature will go up.
Bees may like the heat they get from fixed terrarium heater OR it may be too much heat and they will work to cool hive.
IN ANSWER TO YOUR QUESTION:
"... what is the advantage?"
The advantage of heating a hive with a thermostat and a good heat source is that it can keep the inside temperature of the hive at a constant temperature. I think the more constant the temperature, the better for the bees.
Lower Thermostat setting (04C/39F) = better for winter-sleep (fewer die).
Higher Thermostat setting (18C/65F*) = Better for spring start up (easier brood rearing, less or no brood chilling during fast brood increases versus bee cover).
Inside hive temperatures and bee behavior:
34C-35C/93F-96F Interior Brood Nest Temperature
18C/65F* Outside Brood Nest - Low Reasonable Working Temperature
16C/59F Inside Hive - Bees Huddle In Groups
14C/57F Inside Hive - Bees Enter Cluster
05C/41F Inside Hive - Minimum Body Temperature Of Bees On Outside Of Cluster That Can Still Move
04C/39F Inside Hive - Body Temperature Of Bees On Outside Of Cluster That Can No Longer Move
03C-05C/37F-41F Inside temperature of Canadian indoor honeybee wintering buildings for lowest death rate during winter-sleep (not inside hive temperature)
*(my experience, I have not found this number printed)
A thermostat with a good heat source does NOT heat the whole hive to the same temperature. The temperature in different places in the hive is different... bottom board, inner cover, outside walls, inside cluster... all different.
So where to put the thermostat sensor in the hive that is best for the bees?
This is like asking "where is the best place for a thermostat in a house." In the bedroom? In the kitchen? In the garage? In the family area? I'd say in the family area.
In a heated hive with a thermostat and heat coming from under the screened bottom board, I am guessing the thermostat sensor should be under the frames in the center of the bottom brood box.
Finski, I respect your many years of working with bees and 8 years working with hive heat. You say you have not used thermostats. That is OK. What is your best guess of where to put the thermostat sensor inside the hive that is best for the bees?
I will try to send a photograph of the thermostat and remote sensor in the next post.
Sorry for long post. I am trying to explain myself clearly.
If easier to get into detail, then respond in Finnish and I will have it translated.
Thanks for your help.