Rotating the fan provides the following assurances:
1. The fan assembly actually moves.
2. A confirmation of the absence of an unacceptable amount of ice.
3. You can see the inside of the bypass duct to confirm there is no debris or birds etc.
4. That all the compressor inlet and guide vanes are free from obvious damage and no obvious ingestion has taken place.
5. As the blades pass, they all look equal (ish) and there are no obvious buckles or dings.