Welcome

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You'll find with most bikes that the fan is wired through a temperature sensor switch so that it only comes on when the coolant reaches a certain temperature. Most of the time this is only when parking your bike or sitting still. When you're moving the airflow through the radiator keeps the coolant at a good temperature, when you slow down there's not so much airflow so the fan is there to help keep the engine temperature down.
Some bikes the fan will only run when the ignition is on, some like my 750 don't rely on the key being in the ignition for the fan to run.