Faster access to map data is one of the reason. The are many reasons for hard drive.
Here is info on hard drive, which explains reason for hard drive.
More info in this post
The system will include a 40-gigabyte (GB) hard disk drive, reportedly partitioned as follows (hopefully there will be more room for music and less room for names and addresses when we get final specs): - System 2GB
- Music 3GB
- Entertainment server 8GB
- Navigation map data 12GB
- Telephone (name and address) 10GB
- Reserve 5GB