It creates airflow which evens out the dry throughout the plant. If you aim the fan directly at the plant and it's too strong, you will dry the plant too quickly which will destroy terpenes and make the plant smell like hay. A proper dry should take about 2 weeks, then it's ready for curing in jars/bags.
It's better to hang the whole plant somewhere with low humidity (40-50% - e.g. your grow tent) and have a fan in the room pointing away from the plants to ensure a good airflow and slow dry at around 18-21°C. The same fan you used during the grow cycle will do the job on it's lowest setting, another reason why your grow tent is a good option for drying!
Good luck!