The buoyancy dual force can be used as grid force (in the dynamics nodes) or as particle force (in a particle group).
It generates a force pointing upward according to the temperature available in a temperature (or scalar) channel.
The idea behind it is that hot things rise upwards (buoyancy law).

It is useful to create fire, hot gases, explosions, boiling or any other effect that is based on temperature (or rather the values of a scalar channel because you can link any scalar channel here).