MQTT sensor vs binary sensor

From the name i would think that binary sensor is great for switches - receiveing 0 and 1s only.
But looking at the description binary has the sensor_class (Optional) and just sensor - none. So what is the difference betwwen two and which should be used for example for temp/humidity sensors (mysensors with MQTT)

sensor_class is used for the visualization as stated in the docs. Only sensors can display numerical values.

still a little confusing.
Can you elaborate on an example?
Is it something like you have a sensor item for temperature.
Than a rule which activates a binary sensor if temp goes above 40C?

a binary sensor is like a motion sensor. detection or not.
or a door, open or closed.

a temperature sensor cant be binary.